MNT: Use ReadTheDocs v2 YAML and not conda

[pllim]

1. Upgrade from v1 to v2 ReadTheDocs YAML specifications. See https://docs.readthedocs.io/en/stable/config-file/v2.html
2. Stop using conda to set up the environment. Use pip all around for faster build time and more predictable outcome.

[pllim]

p.s. I can take a stab at this if you want, but I want to make sure everyone is on board first.

[bsipocz]

Sounds like a good plan to me. We need to do point no1 anyway, and the no2 sounds quite reasonable to me, too.

[pllim]

Fixed in #438