Merge pull request projectmesa#972 from rht/pypy

GA: Add pypy3

.github/workflows/python.yml

@@ -20,16 +20,27 @@ jobs:
             python-version: 3.7
           - os: ubuntu-latest
             python-version: 3.8
+          - os: ubuntu-latest
+            python-version: pypy3
+
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+    - uses: actions/cache@v2
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ matrix.python-version }}-${{ hashFiles('Pipfile.lock') }}
+      if: matrix.python-version == 'pypy3'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install codecov black
+        # 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: |