PROTON-2399 Support Python 3.10 (#322)

Using py310 in envlist is necessary on Fedora Rawhide, which might be installed with
this version of Python only.

Using py310 in envlist is supported since tox v3.14.0
diff --git a/python/setup.py.in b/python/setup.py.in
index a83d0d5..697fa70 100644
--- a/python/setup.py.in
+++ b/python/setup.py.in
@@ -277,7 +277,8 @@
                    "Programming Language :: Python :: 3.6",
                    "Programming Language :: Python :: 3.7",
                    "Programming Language :: Python :: 3.8",
-                   "Programming Language :: Python :: 3.9"],
+                   "Programming Language :: Python :: 3.9",
+                   "Programming Language :: Python :: 3.10"],
       cmdclass={
           'configure': Configure,
           'swig': Swig,
diff --git a/python/tox.ini b/python/tox.ini
index 9206d57..f0ae06e 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 # This will be overridden by ctest setting TOXENV for anything but this default
-envlist = pep8,py36,py37,py38,py39
+envlist = pep8,py36,py37,py38,py39,py310
 minversion = 1.7.2
 setupdir = {toxinidir}/dist
 skip_missing_interpreters = True