Docs fail to build

[thangleiter]

Readthedocs sometimes (it's unclear to me why only sometimes) kills the build process due to excessive memory consumption of the conda environment solver.

This could be solved by

• switching to a pip-only build w/o a conda environment. However, in this case qutip needs to be compiled, which at the moment fails because it requires cython at install time (which is not installed by default on the readthedocs docker image). See Installing as external dependency fails if Cython is missing qutip/qutip#1174.
• conda not solving even though an environment.yml file with pinned versions is passed to the command that creates the environment. For now, conda seems to solve the environment despite pinned version numbers. See here for the conda Google groups question.

[thangleiter]

Builds still fail sometimes. Since this is a readthedocs issue, see e.g. readthedocs/readthedocs.org#6710.

[thangleiter]

Seems to have been fixed upstream. Builds working reliably.