readthedocs: Export dev dependencies in requirements.txt

[andrewsomething]

Readthedocs builds are failing with the following error. They rely on requirements.txt for dependencies, not poetry. The theme is only in the dev dependencies. This exports those to requirements.txt as well.

Running Sphinx v5.3.0
making output directory... done
WARNING: html_static_path entry '_static' does not exist
WARNING: sphinx_rtd_theme (< 0.3.0) found. It will not be available since Sphinx-6.0

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 280, in build_main
    args.pdb)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 262, in __init__
    self._init_builder()
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 334, in _init_builder
    self.builder.init()
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/builders/html/__init__.py", line 248, in init
    self.init_templates()
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/builders/html/__init__.py", line 299, in init_templates
    self.theme = theme_factory.create(themename)
  File "/home/docs/checkouts/readthedocs.org/user_builds/pydo/envs/latest/lib/python3.7/site-packages/sphinx/theming.py", line 241, in create
    raise ThemeError(__('no theme named %r found (missing theme.conf?)') % name)
sphinx.errors.ThemeError: no theme named 'sphinx_rtd_theme' found (missing theme.conf?)

Theme error:
no theme named 'sphinx_rtd_theme' found (missing theme.conf?)

[loosla]

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