build: register mock dependency with Bazel

Summary:
We’ve heretofore used the `mock` library without telling Bazel about it.
This isn’t generally a problem when `tensorflow` is installed in the
same virtualenv, because `tensorflow` pulls in `mock`. But tests that
don’t depend on TensorFlow have no build edge to `mock` when imported
into google3’s build system, causing build failures after syncing. This
commit makes the `mock` dependency explict.

We use `mock==1.0.0` rather than the current version (2.0.0) because the
current version depends on `pbr` at runtime in a way that does not play
well with Bazel: <testing-cabal/mock#383>
(It appears to be trying to find some globally registered `setup.cfg`
file from which to read a version identifier.) Rather than vendoring and
patching `mock`, declaring a fake `:expect_mock_installed` target, or
hacking `PBR_VERSION=2.0.0` into the action env, we simply depend on a
saner version of the library, which is also the version used in google3.

Existing `mock` users have been detected and fixed up:

```
$ bazel query "
>     kind(
>         py_.*rule,
>         rdeps(//..., set($(git grep -l 'import mock\($\| \)')), 1)
>     )
> " \
> | awk '{ print "add deps @org_pythonhosted_mock|" $0 }' \
> | buildozer -f - \
> ;
```

Test Plan:
All notf tests now pass in a virtualenv that does not include `mock`:

```
$ virtualenv -q -p python2 ./ve
$ . ./ve/bin/activate
(ve) $ pip install 'absl-py>=0.7.0' 'numpy<2.0,>=1.14.5' boto3==1.9.86 flake8==3.5.0 futures==3.1.1 grpcio==1.6.3 yamllint==1.5.0 >/dev/null 2>&1
(ve) $ python -c 'import mock'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/buildtools/current/sitecustomize/sitecustomize.py", line 152, in SetupPathsAndImport
    return real_import(name, globals, locals, fromlist, level)
ImportError: No module named mock
(ve) $ cd ~/git/tensorboard
(ve) $ bazel test //tensorboard/... --test_tag_filters=support_notf
...
INFO: Build completed successfully, 12 total actions
//tensorboard:lazy_test                                                  PASSED in 0.2s
//tensorboard:lib_test                                                   PASSED in 0.9s
//tensorboard:manager_test                                               PASSED in 0.3s
//tensorboard:plugin_util_test                                           PASSED in 0.5s
//tensorboard:program_test                                               PASSED in 0.8s
//tensorboard/backend:application_test                                   PASSED in 1.0s
//tensorboard/backend:http_util_test                                     PASSED in 0.8s
//tensorboard/backend:json_util_test                                     PASSED in 0.8s
//tensorboard/compat/tensorflow_stub:gfile_test                          PASSED in 0.8s
//tensorboard/summary:summary_test                                       PASSED in 0.8s
//tensorboard/util:platform_util_test                                    PASSED in 0.3s
```

Previously, `manager_test` and `application_test` failed in such an
environment.

wchargin-branch: mock-dep

.travis.yml

@@ -102,7 +102,6 @@ install:
   - pip install flake8==3.5.0
   - pip install futures==3.1.1
   - pip install grpcio==1.6.3
-  - pip install mock==2.0.0
   - pip install moto==1.3.7
   - pip install yamllint==1.5.0
   - |

tensorboard/BUILD

@@ -83,6 +83,7 @@ py_test(
         ":test",
         ":version",
         "//tensorboard/util:tb_logging",
+        "@org_pythonhosted_mock",
         "@org_pythonhosted_six",
     ],
 )
@@ -101,6 +102,7 @@ py_test(
     deps = [
         ":manager",
         "//tensorboard:expect_tensorflow_installed",
+        "@org_pythonhosted_mock",
         "@org_pythonhosted_six",
     ],
     data = [

tensorboard/backend/BUILD

@@ -90,6 +90,7 @@ py_test(
         "//tensorboard/backend/event_processing:event_multiplexer",
         "//tensorboard/plugins:base_plugin",
         "@org_pocoo_werkzeug",
+        "@org_pythonhosted_mock",
     ],
 )
 

tensorboard/plugins/hparams/BUILD

@@ -61,6 +61,7 @@ py_test(
         "//tensorboard:expect_tensorflow_installed",
         "//tensorboard/backend/event_processing:event_accumulator",
         "//tensorboard/backend/event_processing:event_multiplexer",
+        "@org_pythonhosted_mock",
     ],
 )
 
@@ -73,6 +74,7 @@ py_test(
     deps = [
         ":hparams_plugin",
         "//tensorboard:expect_tensorflow_installed",
+        "@org_pythonhosted_mock",
     ],
 )
 
@@ -87,6 +89,7 @@ py_test(
         "//tensorboard:expect_tensorflow_installed",
         "//tensorboard/backend/event_processing:event_accumulator",
         "//tensorboard/backend/event_processing:event_multiplexer",
+        "@org_pythonhosted_mock",
     ],
 )
 

tensorboard/plugins/interactive_inference/BUILD

@@ -39,6 +39,7 @@ py_test(
         "//tensorboard/plugins/interactive_inference/utils:platform_utils",
         "//tensorboard/plugins/interactive_inference/utils:test_utils",
         "@org_pocoo_werkzeug",
+        "@org_pythonhosted_mock",
         "@org_pythonhosted_six",
     ],
 )

tensorboard/plugins/interactive_inference/utils/BUILD

@@ -53,6 +53,7 @@ py_test(
         ":test_utils",
         "//tensorboard:expect_numpy_installed",
         "//tensorboard:expect_tensorflow_installed",
+        "@org_pythonhosted_mock",
         "@org_tensorflow_serving_api",
     ],
 )

third_party/mock.BUILD

@@ -0,0 +1,15 @@
+# Description:
+#   Rolling backport of unittest.mock for all Pythons
+
+licenses(["notice"])  # MIT
+
+exports_files(["LICENSE"])
+
+py_library(
+    name = "org_pythonhosted_mock",
+    srcs = [
+        "mock.py",
+    ],
+    srcs_version = "PY2AND3",
+    visibility = ["//visibility:public"],
+)

third_party/python.bzl

@@ -76,6 +76,17 @@ def tensorboard_python_workspace():
         build_file = str(Label("//third_party:werkzeug.BUILD")),
     )
 
+    http_archive(
+        name = "org_pythonhosted_mock",
+        urls = [
+            "https://mirror.bazel.build/files.pythonhosted.org/packages/85/60/ec8c1af81337bab0caba188b218b6758bc94f125f49062f7c5f0647749d2/mock-1.0.0.tar.gz",
+            "https://files.pythonhosted.org/packages/85/60/ec8c1af81337bab0caba188b218b6758bc94f125f49062f7c5f0647749d2/mock-1.0.0.tar.gz",
+        ],
+        sha256 = "2d9fbe67001d2e8f02692075257f3c11e1b0194bd838c8ce3f49b31fc6c3f033",
+        strip_prefix = "mock-1.0.0",
+        build_file = str(Label("//third_party:mock.BUILD")),
+    )
+
     http_archive(
         name = "org_pythonhosted_six",
         urls = [