fixup! Add github action to run breathe's unit tests

.github/workflows/unit_tests.yml

@@ -7,7 +7,21 @@ jobs:
     strategy:
       matrix:
         python-version: [3.5, 3.6, 3.7, 3.8]
-        sphinx-version: [2.0.0, 2.0.1, 2.1.0, 2.1.1, 2.1.2, 2.2.0, 2.2.1, 2.2.2, 2.3.0, 2.3.1]
+        sphinx-version:
+          - 2.0.0
+          - 2.0.1
+          - 2.1.0
+          - 2.1.1
+          - 2.1.2
+          - 2.2.0
+          - 2.2.1
+          - 2.2.2
+          - 2.3.0
+          - 2.3.1
+          # development branch for sphinx 2.4
+          - git+https://github.com/sphinx-doc/[email protected]
+          # development branch for sphinx 3
+          - git+https://github.com/sphinx-doc/sphinx.git@master
 
     steps:
       - uses: actions/checkout@v2
@@ -22,8 +36,13 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
-      - name: install sphinx
-        run: pip install -Iv Sphinx==${{ matrix.sphinx-version }}
+      - name: install sphinx from PyPI or from git
+        run: |
+          if echo "${{ matrix.sphinx-version }}"|grep -q git; then
+              pip install ${{ matrix.sphinx-version }}
+          else
+              pip install -Iv Sphinx==${{ matrix.sphinx-version }}
+          fi
 
       - name: install dependencies
         run: |