Mock is a test dependency and should not be in install_requires

[NotoriousPyro]

appengine-python-standard/setup.py

Line 21 in 9432aae

"mock>=4.0.3",

https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies

This causes mock to be included as a production dependency in the requirements.txt when using pip-compile:

mock==4.0.3 \
    --hash=sha256:122fcb64ee37cfad5b3f48d7a7d51875d7031aaf3d8be7c42e2bee25044eee62 \
    --hash=sha256:7d3fbbde18228f4ff2f1f119a45cdffa458b4c0dee32eb4d2bb2f82554bac7bc
    # via appengine-python-standard