This ensures they are automatically tagged in PRs for reviews.

…ls (#812)

This is an optional parameter that can be set via the constructor.
It is used to provide the credentials with new tokens and their
expiration time on `refresh()` call.

```
def refresh_handler(request, scopes):
    # Generate a new token for the requested scopes by calling
    # an external process.
    return (
        "ACCESS_TOKEN",
        _helpers.utcnow() + datetime.timedelta(seconds=3600))

creds = google.oauth2.credentials.Credentials(
    scopes=scopes,
    refresh_handler=refresh_handler)
creds.refresh(request)
```

It is useful in the following cases:
- Useful in general when tokens are obtained by calling some
  external process on demand.
- Useful in particular for retrieving downscoped tokens from a
  token broker.

This should have no impact on existing behavior. Refresh tokens
will still have higher priority over refresh handlers.

A getter and setter is exposed to make it easy to set the callable
on unpickled credentials as the callable may not be easily serialized.

```
unpickled = pickle.loads(pickle.dumps(oauth_creds))
unpickled.refresh_handler = refresh_handler
```

Source-Link: googleapis/synthtool@dd05f9d
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:aea14a583128771ae8aefa364e1652f3c56070168ef31beb203534222d842b8b

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

* fix: do not use the GAE APIs on gen2+ runtimes

Currently, this library uses the App Engine API in all environments if
it can be imported successfully. This assumption made sense when the API
was only available on gen1, but this is no longer the case.
See https://github.com/GoogleCloudPlatform/appengine-python-standard

In order to comply with AIP-4115, we must treat GAE gen2+ as a "compute
engine equivalent environment" even if the GAE APIs are importable.
In other words, google.auth.default() must never return an
app_engine.Credental on GAE gen2+.Currently, this library uses the App Engine API in all environments if
it can be imported successfully. This assumption made sense when the API
was only available on gen1, but this is no longer the case.
See https://github.com/GoogleCloudPlatform/appengine-python-standard

In order to comply with AIP-4115, we must treat GAE gen2+ as a "compute
engine equivalent environment" even if the GAE APIs are importable.
In other words, google.auth.default() should not return an
app_engine.Credental on GAE gen2+.

* blacken

Co-authored-by: arithmetic1728 <[email protected]>

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