[python][tests] unused and missing imports

[jameslamb]

Just out of curiosity, I ran this on the codebase tonight:

flake8 --exclude=E501,W503

This PR fixes these warnings from it

./tests/python_package_test/test_engine.py:1860:9: F841 local variable 'iter_min' is assigned to but never used
./tests/python_package_test/test_sklearn.py:304:21: F821 undefined name 'warnings'
./tests/c_api_test/test_.py:4:1: F401 'sys' imported but unused

A couple other warnings are generated but I think they should be ignored. Some thing in outt tests look like "local variable never used" but they're things like this where even if we don't test the value, generating acts as a smoke test:

model = lgb.train(default_params, lgb_train, valid_sets=[lgb_val])

Here is the full log if you're curious:

flake8 logs

./tests/python_package_test/test_engine.py:924:9: F841 local variable 'gbm' is assigned to but never used
./tests/python_package_test/test_engine.py:1024:13: F841 local variable 'gbm' is assigned to but never used
./tests/python_package_test/test_engine.py:1860:9: F841 local variable 'iter_min' is assigned to but never used
./tests/python_package_test/test_engine.py:2102:9: F841 local variable 'model' is assigned to but never used
./tests/python_package_test/test_plotting.py:149:9: F841 local variable 'gbm0' is assigned to but never used
./tests/python_package_test/test_plotting.py:164:9: F841 local variable 'gbm1' is assigned to but never used
./tests/python_package_test/test_sklearn.py:193:9: F841 local variable 'gbm_clone' is assigned to but never used
./tests/python_package_test/test_sklearn.py:304:21: F821 undefined name 'warnings'
./tests/c_api_test/test_.py:4:1: F401 'sys' imported but unused
./python-package/lightgbm/compat.py:28:5: F401 'itertools.izip as zip_' imported but unused
./python-package/lightgbm/compat.py:29:19: F821 undefined name 'basestring'
./python-package/lightgbm/compat.py:30:27: F821 undefined name 'long'
./python-package/lightgbm/compat.py:31:27: F821 undefined name 'long'
./python-package/lightgbm/compat.py:32:14: F821 undefined name 'xrange'
./python-package/lightgbm/compat.py:48:5: F401 'json' imported but unused
./python-package/lightgbm/compat.py:83:5: F401 'matplotlib' imported but unused
./python-package/lightgbm/compat.py:90:5: F401 'graphviz' imported but unused

[StrikerRUS]

Thanks, LGTM!