Add pip pin to dev-requirements

ReadTheDocs installs dev-requirements before requirements.txt.
Use this to reduce version of pip to use older resolver, so
pip install of requirements.txt does not fail on conflicts.

dev-requirements.txt

@@ -9,6 +9,7 @@ flake8==3.8.3
 homebrew-pypi-poet==0.10.0
 isort==4.3.21
 mypy==0.711
+pip<20.3
 pip-check-reqs==2.1.1
 py==1.8.0
 pydocstyle==3.0.0