Find a better way to install packages not on pypi (i.e., those with `git+...` in `requirements.txt`

[rlanzafame]

All direct git downloads in the requirements file are not part of the cached environment, so that's why this affects all branches.

Btw, in the past we had other packages on Gitlab too

Actually it looks like the way that packages that are specified as git repos git+... are checked requires connecting over the internet. It would not happen with the pypi packages. You can see it here with an action from the Manual now that the github sphinx-thebe is reported. This apparently happens every time a build is executed, regardless of cache, so it makes things slower unnecessarily. There has to be a better way to do this in the requirements.txt file

Collecting git+https://github.com/TeachBooks/Sphinx-Thebe (from -r requirements.txt (line 13))
  Cloning https://github.com/TeachBooks/Sphinx-Thebe to /tmp/pip-req-build-n6g9hloi
  Running command git clone --filter=blob:none --quiet https://github.com/TeachBooks/Sphinx-Thebe /tmp/pip-req-build-n6g9hloi
  Resolved https://github.com/TeachBooks/Sphinx-Thebe to commit 94dd8702423894fe2f06417a5cc07948767021bb
  Running command git submodule update --init --recursive -q
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
  Preparing metadata (pyproject.toml): started
  Preparing metadata (pyproject.toml): finished with status 'done'

Originally posted by @rlanzafame in #81

[rlanzafame]

see follow up comment also: #81 (comment)