build: add black formatting to CI

.github/workflows/python-package.yml

@@ -30,8 +30,11 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python setup.py develop
-        python -m pip install flake8 pytest-cov codecov vcrpy
+        python -m pip install flake8 pytest-cov codecov vcrpy black
         tools/install_pandas.sh
+    - name: black
+      run: |
+        black --check tiingo
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names