Update compatible releases and Python==3.9

This allows running tests on Python 3.9.0 and update the Sphinx releases
with all latest 3.x versions, as well as the proper minor development
tag.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

.github/workflows/unit_tests.yml

@@ -6,12 +6,14 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
         sphinx-version:
           - 3.0.4
           - 3.1.2
-          - 3.2.0
-          - git+https://github.com/sphinx-doc/sphinx.git@3.3.x
+          - 3.2.1
+          - 3.3.1
+          - 3.4.3
+          - git+https://github.com/sphinx-doc/sphinx.git@3.4.x
           - git+https://github.com/sphinx-doc/sphinx.git@3.x
           # master (Sphinx 4) will require at least Python 3.6, so disable it for now
           #- git+https://github.com/sphinx-doc/sphinx.git@master