chore: use Python 3.8 for docs / docfx sessions (#125)

Works around recent changes to the Kokoro 'docs-presubmt' configuration.

See: googleapis/python-api-core#224

Also, pin sphinx version (`docs` / `docfx`) and use 'gcp-sphinx-docfx-yaml' (`docfx`).

noxfile.py

@@ -106,13 +106,13 @@ def cover(session):
     session.run("coverage", "erase")
 
 
-@nox.session(python=DEFAULT_PYTHON_VERSION)
+@nox.session(python="3.8")
 def docs(session):
     """Build the docs for this library."""
 
     session.install(".", "grpcio >= 1.8.2", "grpcio-gcp >= 0.2.2")
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark")
+    session.install("sphinx==4.0.1", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
@@ -129,12 +129,14 @@ def docs(session):
     )
 
 
-@nox.session(python=DEFAULT_PYTHON_VERSION)
+@nox.session(python="3.8")
 def docfx(session):
     """Build the docfx yaml files for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark", "sphinx-docfx-yaml")
+    session.install(
+        "sphinx==4.0.1", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml",
+    )
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(