.github/workflows/unit_tests.yml: Update it

Only keep the last two major versions for sphinx and drop 4.x. In addition
sphinx 6.0 dropped support for python 3.7, so we do the same.

Both changes allow us to remove all matrix excludes.

.github/workflows/unit_tests.yml

@@ -7,33 +7,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ['3.7', '3.8', '3.9', '3.10']
+        python-version: ['3.8', '3.9', '3.10', '3.11']
         sphinx-version:
-          - '4.0.3'
-          - '4.1.2'
-          - '4.2.0'
-          - '4.3.2'
-          - '4.5.0'
           - '5.0.2'
           - '5.1.1'
+          - '5.2.3'
           - '5.3.0'
           - '6.1.3'
           - git+https://github.com/sphinx-doc/sphinx.git@master
-        exclude:
-          # avoid bug in following configurations
-          # sphinx/util/typing.py:37: in <module>
-          #     from types import Union as types_Union
-          # ImportError: cannot import name 'Union' from 'types'
-          - python-version: '3.10'
-            sphinx-version: '4.0.3'
-          - python-version: '3.10'
-            sphinx-version: '4.1.2'
-
-          # Sphinx has removed support for Python 3.7, Breathe will follow.
-          - python-version: '3.7'
-            sphinx-version: git+https://github.com/sphinx-doc/sphinx.git@master
-          - python-version: '3.7'
-            sphinx-version: '6.1.3'
 
     steps:
       - uses: actions/checkout@v2