Enable Python 3.10

Author: awdeorio

tox.ini

@@ -1,6 +1,6 @@
 # Local host configuration with one Python 3 version
 [tox]
-envlist = py36, py37, py38, py39
+envlist = py36, py37, py38, py39, py310
 
 # GitHub Actions configuration with multiple Python versions
 # https://github.com/ymyzk/tox-gh-actions#tox-gh-actions-configuration
@@ -10,6 +10,7 @@ python =
   3.7: py37
   3.8: py38
   3.9: py39
+  3.10: py310
 
 # Run unit tests
 # HACK: Pydocstyle fails to find tests.  Invoke a shell to use a glob.