ReadTheDocs build is eagerly installing dependencies, leading to unpredictable builds

[nhoening]

Our docs are not building on ReadTheDocs: https://readthedocs.org/projects/flexmeasures/builds/20795953/

Locally this doesn't happen for me. I investigated, and the error seems to indicate we're installing flask-marshmallow 0.15 on RTD which has an incompatibility (see marshmallow-code/flask-marshmallow#262). It is fixed to 0.14 in app.txt.

After we installed our pinned dependencies into the RTD environment, we pip-install flexmeasures (see .readthedocs.yaml lines 20ff, and on RTD this leads to the command python -m pip install --upgrade --upgrade-strategy eager --no-cache-dir).

Since #670, this uses `app.in.

A possible solution is discussed here and reported here.

Let's try that first.