Add black and mypy to requirements.txt

They are conditional on cpython because currently neither tool can be
installed in a pypy environment.

psf/black#727
python/typed_ast#111

Author: jpgrayson

.github/workflows/ci.yml

@@ -91,7 +91,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install -U pip
-        python -m pip install -e . -r requirements.txt black mypy
+        python -m pip install -e . -r requirements.txt
     - name: Lint
       run: |
         make lint

requirements.txt

@@ -1,5 +1,7 @@
+black >= 19.10b0; implementation_name == "cpython"
 flake8
 isort
+mypy >= 0.770; implementation_name == "cpython"
 pytest
 pytest-cov
 setuptools_scm