use conda on readthedocs

Conda support was recently added to readthedocs. It looks like it's a lot easier to use conda than to mock unneeded dependencies. It should also make it possible for RTD to execute pvlib code and generate figures, which would make it easier for us to write better docs.

Here's the [rtd conda doc page](https://docs.readthedocs.org/en/latest/conda.html). 
