run black only on ubuntu 3.8

.github/workflows/python.yml

@@ -37,18 +37,16 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        # We pin black's version until either PyPy3 becomes Python 3.8 or black
-        # removes its typed-ast dependency. See
-        # https://github.com/psf/black/issues/727
-        pip install codecov black==19.3b0
         pip install .[dev]
     - name: Lint with flake8
       run: |
         # Use settings from mesas .flake8 file
         flake8 . --count --show-source --statistics
     - name: Lint with black
+      if: matrix.python-version == '3.8'
       run: |
-        black --check --exclude=mesa/cookiecutter-mesa/*
+        pip install black
+        black --check --exclude=mesa/cookiecutter-mesa/* .
     - name: Test with pytest
       run: |
         pytest --cov=mesa tests/ --cov-report=xml