ReadTheDocs fails at environment setup

[bollwyvl]

For some time, it looks like the ReadTheDocs build has been failing with:

conda env create --quiet --name latest --file docs/environment.yml
Collecting package metadata: ...working... done
Solving environment: ...working... Killed

Command killed due to excessive memory consumption 

While the environment.yml in question might work (py35, etc), it should probably be updated, but will still be very heavy.

Some (not necessarily mutually-exclusive) approaches:

• needs RTD perms
  • turn on per-PR builds, as mentioned by @willingc so it gets brought to contributors attention more quickly
  • request the CONDA_USES_MAMBA feature flag
• doesn't need RTD perms
  • capture the full environment spec with conda-lock and use that during RTD builds
  • because not solving is even faster than mamba

Given the above (and your humble issue reporter not having or particularly wanting RTD perms), but being a something of a conda nerd, I'll try a PR with the last option while we consider/wait for the other options to become a reality.