build: Use python 3.10 for docs session

docs test container now requires python 3.10. Related: googleapis/python-bigquery#2058

Author: daniel-sanche

noxfile.py

@@ -367,7 +367,7 @@ def cover(session):
     session.run("coverage", "erase")
 
 
-@nox.session(python="3.9")
+@nox.session(python="3.10")
 def docs(session):
     """Build the docs for this library."""