Remove Python 3.10 tests, logging failure here

The dbus module fails import with an Import Error - not finding
collections Sequence.

Logged upstream at
https://gitlab.freedesktop.org/dbus/dbus-python/-/issues/37

Author: davesteele

.github/workflows/testnox.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10.0-beta.3"]
+        python-version: ["3.6", "3.7", "3.8", "3.9"]
     steps:
     - name: Check out
       uses: actions/checkout@v2