Use quotes for python version for github workflows

Fix GitHub workflow failures by using quotes for python versions.
It seems that adding `3.10` as a number is transformed then to `3.1`
which as a result is translated to Python version 3.1 instead of Python
version 3.10.
This seems to work for other projects as well:
https://github.com/MasoniteFramework/masonite4/blob/master/.github/workflows/pythontest.yml
https://github.com/python-pillow/Pillow/blob/main/.github/workflows/test-windows.yml
https://github.com/PyGithub/PyGithub/blob/master/.github/workflows/ci.yml

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       # Run regular TUF tests on each OS/Python combination, plus special tests
       # (sslib master) and linters on Linux/Python3.x only.
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
         os: [ubuntu-latest, macos-latest, windows-latest]
         toxenv: [py]
         include: