chore: Remove support for Python 3.6

[clundin25]

No urgency to merge this, but I do want to use some features from Python 3.7.

3.6 has been EOL for sometime, and the core library has dropped support https://github.com/googleapis/python-api-core.

[parthea]

Python 3.7 is also unsupported in the python community. https://devguide.python.org/versions/#unsupported-versions . I'd recommend dropping both at the same time. Could we consider raising a deprecation warning in the next month notifying users that support for both 3.6 and 3.7 will be dropped on a specific date? We have a similar proposal in google-cloud-python which will be sent for review soon: googleapis/google-cloud-python#11437

Only users who are running the latest version of google-auth, using runtime versions which are unsupported, would receive the warnings, and the warning should only appear once per session. For example, users running python 3.8+ won't see any warnings.

If we have Google projects requiring python 3.7, then let's work together to figure out an acceptable date when support can be dropped across these project and communicate the date in advance via the README (and potentially also through deprecation warnings).

Please file an issue in this repo if you would consider adding a feature to raise a deprecation warning before dropping a language runtime and we can continue the discussion in the issue.

[clundin25]

Historically I think gcloud has been the main reason to keep older versions, but now that we only support Python3 it might be easier to move faster.

Could we write a document that describes what versions we support and for how long? That might scale better than adding deprecations. Something akin to Go's version policy.