modernize read the docs setup

[dhellmann]

• update the version of python used
• add web hook for building docs on PRs
• revert python 3.7 support changes from pin python version in doc build test #47 and doc build take version from git when package not installed #48

[dhellmann]

https://readthedocs.org/projects/virtualenvwrapper/builds/19460162/ shows the web hook working.

[dhellmann]

#50 is merged and this work is done