Enable and fix tests on Python 3.10

from __future__ annotations are the default on Python 3.10 so the tricks
we used to keep the test suite nice can no longer be applied.

CI addition to make sure it stays compatible.

Author: jstasiak

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9, pypy3]
+        python-version: [3.6, 3.7, 3.8, 3.9, 3.10.0-alpha.5, pypy3]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}