feat: expose library version at `google.auth.__version` (#683)

Move the version from `setup.py` to `google/auth/version.py`.  Same as https://github.com/googleapis/python-api-core/pull/80.  (see https://github.com/googleapis/python-api-core/issues/27 for motivation). This is option 3 in https://packaging.python.org/guides/single-sourcing-package-version/.

This unblocks a version check I'd like to add in https://github.com/googleapis/python-api-core/pull/134.

Usage:

```py
>>> import google.auth
>>> google.auth.__version__
'1.25.0'
```