* test: use self.MOCK_CREDENTIALS in tests

* Update test_discovery.py

* Update test_discovery.py

The GAE memcache API isn't supported in python3. This change gives
callers a new way to ensure that googleapiclient doesn't even try
to import and use this legacy API.

Fixes the date of when the global batch endpoint will be turned down, per the extension outlined in https://developers.googleblog.com/2018/03/discontinuing-support-for-json-rpc-and.html.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/google-api-python-client/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes #905 🦕

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/google-api-python-client/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

* updated CHANGELOG.md [ci skip]

* updated setup.py [ci skip]

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>