Add make help

[hugovk]

Add make help similar to the one in the PEPs repo.

❯ make help
Use `make <target>' where <target> is one of
 venv              Create a venv with necessary tools
 clean             Remove the venv
 regen-requirements Regenerate pinned requirements file
 check             Validate buildbot master configuration
 update-master     Pull updates, upgrade, check config, and start master
 start-master      Start the buildbot master
 restart-master    Restart the buildbot master
 stop-master       Stop the buildbot master
 git-update-requirements Create a branch with regenerated requirements