Add Python 3.10 testing.

Author: QuLogic

.github/workflows/tests.yml

@@ -41,6 +41,11 @@ jobs:
             python-version: 3.9
             extra-requirements: '-r requirements/testing/extra.txt'
             XVFB_RUN: xvfb-run -a
+          - os: ubuntu-20.04
+            python-version: '3.10-dev'
+            # Re-add this when extra dependencies have wheels.
+            # extra-requirements: '-r requirements/testing/extra.txt'
+            XVFB_RUN: xvfb-run -a
           - os: macos-latest
             python-version: 3.8
             XVFB_RUN: ""