Cherry pick cleanups from #499.

…p/templates/python_library/.kokoro (#501)

Source-Link: https://togithub.com/googleapis/synthtool/commit/bb171351c3946d3c3c32e60f5f18cee8c464ec51
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:f62c53736eccb0c4934a3ea9316e0d57696bb49c1a7c86c726e9bb8a2f87dadf

Source-Link: googleapis/synthtool@eb78c98
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:8a5d3f6a2e43ed8293f34e06a2f56931d1e88a2694c3bb11b15df4eb256ad163

…501)

* Repo clean-up

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Add README.md

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Rename samples/README.md to samples/snippets/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

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

There was some refactoring, and the bigtable_v2/proto directory no longer exists.
Updated the links to the correct ones, and used permalink.

* updated CHANGELOG.md [ci skip]

* updated setup.py

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

* Sets cache support to false, removes upper req limit

* fix: adds timestamp to ensure tests pass successfully

* fix: updates tests to account for UTC

* restores path in nox session

…lates/python_library/.kokoro (#501)

Source-Link: googleapis/synthtool@9367caa
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:2f155882785883336b4468d5218db737bb1d10c9cea7cb62219ad16fe248c03c

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Daniel Sanche <sanche@google.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Closes #421.

Supersedes #496.

* This patch for </issues/501> includes the following fixes:

- The access token is always set to `None`, so the fix involves using (the access) `token` from the saved JSON credentials file.
- For refresh needs, `expiry` also needs to be saved via `to_json()`.
    - DUMP: As `expiry` is a `datetime.datetime` object, serialize to `datetime.isoformat()` in the same [`oauth2client` format](https://github.com/googleapis/oauth2client/blob/master/oauth2client/client.py#L55) for consistency.
    - LOAD: Add code to restore `expiry` back to `datetime.datetime` object when imported.
    - LOAD: If `expiry` was unsaved, automatically set it as expired so refresh takes place.
- Minor `scopes` updates
    - DUMP: Add property for `scopes` so `to_json()` can grab it
    - LOAD: `scopes` may be saved as a string instead of a JSON array (Python list), so ensure it is Sequence[str] when imported.