Add support for Python 3.10

Author: hugovk

.travis.yml

@@ -10,6 +10,7 @@ python:
   - "3.7"
   - "3.8"
   - "3.9"
+  - "3.10-dev"
 
 install:
   - pip install -U pip setuptools  # https://github.com/pypa/virtualenv/issues/1630

pyproject.toml

@@ -19,6 +19,7 @@ classifiers = [
   "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",
   "Programming Language :: Python :: Implementation :: PyPy",
   "Topic :: Security :: Cryptography",

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 # Environment changes have to be manually synced with '.travis.yml'.
-envlist = py36,p37,p38,p39
+envlist = py36,p37,p38,p39,py310
 
 [pytest]
 addopts = -v --cov rsa --cov-report term-missing