Add developer requirements to setup.py

[peanutfun]

Changes proposed in this PR:

• Remove env_developer.yml and move the developer requirement specifications into extras in setup.py.

  setup.py now defines three extras, doc for the documentation, test for the test tools, and a convenience extra dev which combines the two.

• Remove pip specs from env_climada.yml. Pip will now only be called once and install all remaining packages when Climada is installed.

• Switch from nbsphinx to myst_nb for parsing Jupyter Notebooks for the documentation.

  We previously specified both packages as requirements but only used nbsphinx. However, nbsphinx requires pandoc which cannot be installed via pip. To remove the pandoc requirement from the conda environment, I switched to myst-nb, which works the same way, can be installed via pip, and seems to support more features. From what I can see, this did not change anything in the display of the notebooks (reviewer please double check that!)

• Update .readthedocs.yml to install the docs requirements from setup.py. Alternatively, the env_doc.yml could be updated

This PR fixes #696

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• [ ] Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
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[peanutfun]

I just noticed that math does not render properly, trying to fix that now...

[peanutfun]

Math is rendering now. However, the "Contents" links in the tutorials broke (for the online documentation only). This is because myst uses a different anchor slug structure than nbsphinx. I think we can leave it as is.

https://github.com/CLIMADA-project/climada_python/pull/712/files#diff-cfbe05e73a29504cae64178b403c69f5c532ccf3abeb4e14c66b96e137ca642c shows an example how to fix them

[chahank]

The instructions look clear. @emanuel-schmid : could you please review for functionality?

[emanuel-schmid]

Everything could be so nice - but, alas!, Windows! 🙄

climada_python>python -m pip install -e '.\[dev]'
ERROR: '.\[dev]' is not a valid editable requirement. It should either be a path to a local project or a VCS URL (beginning with bzr+http, bzr+https, bzr+ssh, bzr+sftp, bzr+ftp, bzr+lp, bzr+file, git+http, git+https, git+ssh, git+git, git+file, hg+file, hg+http, hg+https, hg+ssh, hg+static-http, svn+ssh, svn+http, svn+https, svn+svn, svn+file)

Let's see what google can tell me about this one...

[emanuel-schmid]

Actually quite easy:

pip install -e "./[dev]"