fix invalid specifier that was breaking example notebooks & pip-installing from GitHub/sdist. Also update readthedocs configuration files and remove testing on python 2.7 due to breaking changes in readthedocs and GitHub actions.

fix invalid specifier that was breaking example notebooks & pip-installing from GitHub/sdist. Also update readthedocs configuration files and remove testing on python 2.7 due to breaking changes in readthedocs and GitHub actions. #90