Add support for python 3.10

Python 3.10 is released on October 4-th 2021 and it seems
logical to add support for it as it doesn't require any major effort
from the project.

For reference read:
https://www.python.org/downloads/release/python-3100/

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       # Run regular TUF tests on each OS/Python combination, plus special tests
       # (sslib master) and linters on Linux/Python3.x only.
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
         os: [ubuntu-latest, macos-latest, windows-latest]
         toxenv: [py]
         include:

setup.py

@@ -101,6 +101,7 @@
     'Programming Language :: Python :: 3.7',
     'Programming Language :: Python :: 3.8',
     'Programming Language :: Python :: 3.9',
+    'Programming Language :: Python :: 3.10',
     'Programming Language :: Python :: Implementation :: CPython',
     'Topic :: Security',
     'Topic :: Software Development'

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = lint,py{36,37,38,39}
+envlist = lint,py{36,37,38,39,3.10}
 skipsdist = true
 
 [testenv]