Update Python 3 version support

[markcampanelli]

https://github.com/pvlib/pvlib-python/tree/v0.6.3 lists Python 3.4-7 support (see https://github.com/pvlib/pvlib-python/blob/v0.6.3/setup.py). pvfactors is listed as optional, but is called out in the CI requirements for the Python 3.5-7 runs, but Python 3.4 is not included (e.g., https://github.com/pvlib/pvlib-python/blob/v0.6.3/ci/requirements-py34.yml vs. https://github.com/pvlib/pvlib-python/blob/v0.6.3/ci/requirements-py37.yml).

pvfactors should be more clear about which Python 3 versions are supported in the CLASSIFIERS variable in https://github.com/SunPower/pvfactors/blob/master/setup.py (e.g., more than just 'Programming Language :: Python :: 3.6'). It would also be worth checking that the dependency ranges in https://github.com/SunPower/pvfactors/blob/master/requirements.txt are indeed compatible with those Python 3 versions. (It might be nice to be more specific about Python 2 versions as well.)

[markcampanelli]

@anomam Do you know what the lower bound on pandas should now be?

https://github.com/SunPower/pvfactors/blob/master/requirements.txt#L3

[anomam]

Thanks for this @markcampanelli , I think that the lower bound should be the same as in pvlib, because the only functions in pvfactors that really depend on pandas are in the irradiance models, which use pvlib. I'm thinking of just removing the lower bound on that line (and maybe a few other lines as well)