feat: add support for Python 3.10

CONTRIBUTING.rst

@@ -19,7 +19,7 @@ A few notes on making changes to ``google-auth-library-python``.
   using ``nox -s docgen``.
 
 - The change must work fully on the following CPython versions:
-  3.6, 3.7, 3.8, 3.9 across macOS, Linux, and Windows.
+  3.6, 3.7, 3.8, 3.9, 3.10 across macOS, Linux, and Windows.
 
 - The codebase *must* have 100% test statement coverage after each commit.
   You can test coverage via ``nox -e cover``.

noxfile.py

@@ -86,7 +86,7 @@ def blacken(session):
     session.run("black", *BLACK_PATHS)
 
 
-@nox.session(python=["3.6", "3.7", "3.8", "3.9"])
+@nox.session(python=["3.6", "3.7", "3.8", "3.9", "3.10"])
 def unit(session):
     constraints_path = str(
         CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"

setup.py

@@ -63,6 +63,7 @@
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",