ci: convert workflow file to use tox to run mypy

Convert workflow file to use tox to run mypy on the latest version of
python as well as python 3.9 (to avoid incompatible uses).

Signed-off-by: John Mulligan <jmulligan@redhat.com>

Author: phlogistonjohn

.github/workflows/main.yml

@@ -44,6 +44,5 @@ jobs:
           tox -e ruff-check
       - name: Type Checking (mypy)
         run: |
-          python3 -m mypy --version
-          python3 -m mypy varlink/
+          tox -e py3-mypy,py39-mypy
     runs-on: ubuntu-latest