chore: use Python 3.8 for docs / docfx sessions

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

See: googleapis/python-api-core#224

Author: tseaver

noxfile.py

@@ -106,7 +106,7 @@ 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."""
 
@@ -129,7 +129,7 @@ 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."""