CI: test on released Python 3.10

Author: akx

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04, windows-2019, macos-10.15]
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10-dev, pypy3]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "pypy3"]
         exclude:
           - os: windows-2019
             python-version: pypy3

tox.ini

@@ -23,4 +23,4 @@ python =
     3.7: py37
     3.8: py38
     3.9: py39
-    3.10-dev: py310
+    3.10: py310