#818: unify RFC 3339 timestamp format strubg

)

Fixes #835.

Breakdown by name that was originally added in #824
- `auth`: `from google import auth` -> `import google.auth`
- `credentials`: `from google.auth import credentials` -> `from google.auth import credentials as ga_credentials`
- `exceptions`: `from google.api_core import exceptions` -> `from google.api_core import exceptions as core_exceptions`
- `future`: skipped, as it is only used in the [generated tests](https://github.com/googleapis/gapic-generator-python/search?q=%22import+future%22) and has a low chance of colliding
- `options` `from google.iam.v1 import options_pb2 as options` -> `from google.iam.v1 import options_pb2`
- `policy` `from google.iam.v1 import policy_pb2 as policy` -> `from google.iam.v1 import policy_pb2`
- `math` skipped as it is only used in [generated tests](https://github.com/googleapis/gapic-generator-python/search?q=%22import+math%22)

For `options` and `policy` there is a small change to `gapic/schema/metadata.py` to not alias `_pb2` types

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

…835)

These are _very_ lenient minimums; they represent the oldest
versions I could find on pypi for which the tests all pass.

Also cap the major versions for pymemcache and redis to
protect against hypothetical future breaking changes.
(pytz it doesn't make sense to do that for)

Include them in the constraints file for 3.7 as well for
testing purposes.

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

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

Co-authored-by: Dan Lee <71398022+dandhlee@users.noreply.github.com>

🤖 I have created a release \*beep\* \*boop\*
---
## [2.0.0](https://www.github.com/googleapis/google-auth-library-python/compare/v2.0.0-b1...v2.0.0) (2021-08-16)

### ⚠ BREAKING CHANGES
* drop support for Python 2.7 ([#778](https://www.github.com/googleapis/google-auth-library-python/issues/778)) ([560cf1e](https://www.github.com/googleapis/google-auth-library-python/commit/560cf1ed02a900436c5d9e0a0fb3f94b5fd98c55))

### Features

* service account is able to use a private token endpoint ([#835](https://www.github.com/googleapis/google-auth-library-python/issues/835)) ([20b817a](https://www.github.com/googleapis/google-auth-library-python/commit/20b817af8e202b0331998e5abde4e2a5aab51f9a))


### Bug Fixes

* downscoping documentation bugs ([#830](https://www.github.com/googleapis/google-auth-library-python/issues/830)) ([da8bb13](https://www.github.com/googleapis/google-auth-library-python/commit/da8bb13c1349e771ffc2e125256030495c53d956))
* Fix missing space in error message. ([#821](https://www.github.com/googleapis/google-auth-library-python/issues/821)) ([7b03988](https://www.github.com/googleapis/google-auth-library-python/commit/7b039888aeb6ec7691d91c9afce182b17f02b1a6))


### Documentation

* update user guide/references for downscoped creds ([#827](https://www.github.com/googleapis/google-auth-library-python/issues/827)) ([d1840dc](https://www.github.com/googleapis/google-auth-library-python/commit/d1840dcdcd03dfd7fdfa81d08da68402f6f8b658))
---


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