test: use python 3.10 for docs (#305)

Author: Linchin

noxfile.py

@@ -376,7 +376,7 @@ 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."""