Add a pre-commit config

.pre-commit-config.yaml

+repos:
+  - repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+      - id: black
+        language_version: python3
+
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.8.4
+    hooks:
+      - id: flake8

requirements.txt

 -e ./
-flake8
+flake8==3.8.4
 pep8
 pyflakes
 black==20.8b1; platform.python_implementation!='PyPy'
@@ -13,3 +13,4 @@ versioneer
 jax; python_version > '3.6'
 jaxlib; python_version > '3.6'
 diff-cover
+pre-commit