Repository: googleapis/google-auth-library-python

Issue:
Normalize nox / docs builds
## `docgen`

` `noxfile.py` contains a `docgen` session which, when run, delete's / overwrites some files in the `docs/reference` tree:

```bash
$ nox -re docgen
nox > Running session docgen
nox > Re-using existing virtual environment at .nox/docgen.
nox > pip install flask freezegun mock oauth2client pyopenssl pytest pytest-cov pytest-localserver requests urllib3 cryptography responses grpcio
nox > pip install sphinx
nox > pip install .
nox > rm -r docs/reference
nox > Warning: rm is not installed into the virtualenv, it is located at /bin/rm. This might cause issues! Pass external=True into run() to silence this message.
nox > sphinx-apidoc --output-dir docs/reference --separate --module-first google
Creating file docs/reference/google.rst.
Creating file docs/reference/google.auth.rst.
Creating file docs/reference/google.auth.app_engine.rst.
Creating file docs/reference/google.auth.credentials.rst.
Creating file docs/reference/google.auth.environment_vars.rst.
Creating file docs/reference/google.auth.exceptions.rst.
Creating file docs/reference/google.auth.iam.rst.
Creating file docs/reference/google.auth.impersonated_credentials.rst.
Creating file docs/reference/google.auth.jwt.rst.
Creating file docs/reference/google.auth.compute_engine.rst.
Creating file docs/reference/google.auth.compute_engine.credentials.rst.
Creating file docs/reference/google.auth.crypt.rst.
Creating file docs/reference/google.auth.crypt.base.rst.
Creating file docs/reference/google.auth.crypt.es256.rst.
Creating file docs/reference/google.auth.crypt.rsa.rst.
Creating file docs/reference/google.auth.transport.rst.
Creating file docs/reference/google.auth.transport.grpc.rst.
Creating file docs/reference/google.auth.transport.mtls.rst.
Creating file docs/reference/google.auth.transport.requests.rst.
Creating file docs/reference/google.auth.transport.urllib3.rst.
Creating file docs/reference/google.oauth2.rst.
Creating file docs/reference/google.oauth2.credentials.rst.
Creating file docs/reference/google.oauth2.id_token.rst.
Creating file docs/reference/google.oauth2.service_account.rst.
Creating file docs/reference/modules.rst.
nox > Session docgen was successful.
[/home/tseaver/projects/agendaless/Google/src/google-auth] 
$ git stat
On branch master
Your branch is up to date with 'origin/master'.

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	deleted:    docs/reference/google.auth.credentials_async.rst
	deleted:    docs/reference/google.auth.jwt_async.rst
	modified:   docs/reference/google.auth.rst
	deleted:    docs/reference/google.auth.transport.aiohttp_requests.rst
	modified:   docs/reference/google.auth.transport.rst
	deleted:    docs/reference/google.oauth2.credentials_async.rst
	modified:   docs/reference/google.oauth2.rst
	deleted:    docs/reference/google.oauth2.service_account_async.rst
```

I recommend that we delete this section, as a fossil of an approach we no longer mean to support.

## `docs` session

The `docs` session triggers the build via `make` (and the `docs/Makefile`), rather than running `sphinx-build` directly as in other projects.  This choice is unique across all our projects, and means that we don't enforce
the same specs here (e.g., Sphinx warnings are not treated as errors).

I recommend we delete the `docs/Makefile` and normalize the `docs` session, fixing any Sphinx warnings found.


Discussion:
@busunkim96 fixed the second half of this issue in b1a12d2d.

Resolving patch:
diff --git a/noxfile.py b/noxfile.py
--- a/noxfile.py
+++ b/noxfile.py
@@ -115,23 +115,6 @@ def cover(session):
     session.run("coverage", "report", "--show-missing", "--fail-under=100")
 
 
-@nox.session(python="3.7")
-def docgen(session):
-    session.env["SPHINX_APIDOC_OPTIONS"] = "members,inherited-members,show-inheritance"
-    session.install("-r", "testing/requirements.txt")
-    session.install("sphinx")
-    session.install("-e", ".")
-    session.run("rm", "-r", "docs/reference")
-    session.run(
-        "sphinx-apidoc",
-        "--output-dir",
-        "docs/reference",
-        "--separate",
-        "--module-first",
-        "google",
-    )
-
-
 @nox.session(python="3.8")
 def docs(session):
     """Build the docs for this library."""


Test patch: