Bumps Python version for black and flake checks to 3.9 to ensure tool…

… compatibility

.github/workflows/unit-tests.yaml

@@ -18,9 +18,6 @@ jobs:
         os: [ubuntu-20.04]
         # TODO: add pypy2, pypy3
         python-version: [3.7, 3.8, 3.9, "3.10", "3.11"]
-        exclude:
-          - os: macos-latest
-            python-version: [3.6]
 
     steps:
     - uses: actions/checkout@v2
@@ -41,15 +38,15 @@ jobs:
         python -m pip list
 
     - name: Update Black
-      if: ${{ matrix.python-version == 3.7 }}
+      if: ${{ matrix.python-version == 3.9 }}
       run: |
         pip install flake8-pytest-importorskip
         pip install --upgrade click==8.0.4
         pip install black==21.12b0
         pip install flake8==4.0.1
 
     - name: Lint and Format Check with Flake8 and Black
-      if: ${{ matrix.python-version == 3.7 }}
+      if: ${{ matrix.python-version == 3.9 }}
       run: |
         black --diff --check .
         flake8