ci: Add Python 3.10

Needs a couple of follow-up commits for workarounds, which hopefully
aren't needed anymore at a later point.

Fixes #6068

.github/workflows/ci.yml

@@ -120,6 +120,10 @@ jobs:
           - testenv: py39-pyqt515-cov
             os: ubuntu-20.04
             python: 3.9
+          ### PyQt 5.15 (Python 3.10)
+          - testenv: py310-pyqt515
+            os: ubuntu-20.04
+            python: 3.10-dev
           ### macOS: PyQt 5.15 (Python 3.7 to match PyInstaller env)
           - testenv: py37-pyqt515
             os: macos-10.15

tox.ini

@@ -23,6 +23,7 @@ basepython =
     py37: {env:PYTHON:python3.7}
     py38: {env:PYTHON:python3.8}
     py39: {env:PYTHON:python3.9}
+    py310: {env:PYTHON:python3.10}
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-tests.txt