Fix pip import path in 19.3

This patch was provided by hgibsonqb
#1272 (comment)

chalice/compat.py

@@ -11,11 +11,16 @@ def pip_import_string():
     # type: () -> str
     import pip
     pip_major_version = pip.__version__.split('.')[0]
+    pip_minor_version = pip.__version__.split('.')[1]
     # Pip moved its internals to an _internal module in version 10.
     # In order to be compatible with version 9 which has it at at the
     # top level we need to figure out the correct import path here.
     if pip_major_version == '9':
         return 'from pip import main'
+    # Pip changed their import structure again in 19.3
+    # https://github.com/pypa/pip/commit/09fd200
+    elif pip_major_version == '19' and int(pip_minor_version) >= 3:
+        return 'from pip._internal.main import main'
     else:
         return 'from pip._internal import main'