Readthedocs CI builds failing to setup speclite; use_2to3 is invalid

[rrjbca]

Recent readthedocs CI builds have started failing and are blocking pull requests. The last build on main was successful yesterday at ~16:30 BST (see HERE), yet recent PRs with seemingly unrelated changes are now failing to setup speclite. See the full logs (e.g. HERE) for details. Relevant extract:

Collecting speclite>=0.11
  Downloading speclite-0.13.tar.gz (813 kB)
    ERROR: Command errored out with exit status 1:
     command: /home/docs/checkouts/readthedocs.org/user_builds/skypy/envs/487/bin/python -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/setup.py'"'"'; __file__='"'"'/tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-eaypa02d
         cwd: /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/
    Complete output (11 lines):
    /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/astropy_helpers/astropy_helpers/setup_helpers.py:163: AstropyDeprecationWarning: The package argument to generate_version_py has been deprecated and will be removed in future. Specify the package name in setup.cfg instead
      'the package name in setup.cfg instead', AstropyDeprecationWarning)
    /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/astropy_helpers/astropy_helpers/setup_helpers.py:168: AstropyDeprecationWarning: The version argument to generate_version_py has been deprecated and will be removed in future. Specify the version number in setup.cfg instead
      'the version number in setup.cfg instead', AstropyDeprecationWarning)
    /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/astropy_helpers/astropy_helpers/setup_helpers.py:174: AstropyDeprecationWarning: The release argument to generate_version_py has been deprecated and will be removed in future. We now use the presence of the "dev" string in the version to determine whether this is a release
      'determine whether this is a release', AstropyDeprecationWarning)
    /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/astropy_helpers/astropy_helpers/setup_helpers.py:188: AstropyDeprecationWarning: Specifying the package name using the "package_name" option in setup.cfg is deprecated - use the "name" option instead.
      'option instead.', AstropyDeprecationWarning)
    /tmp/pip-install-7_pv_l_b/speclite_70ca1ded107648198566ad6dd02280a0/astropy_helpers/astropy_helpers/version_helpers.py:266: AstropyDeprecationWarning: Specifying the package name using the "package_name" option in setup.cfg is deprecated - use the "name" option instead.
      'option instead.', AstropyDeprecationWarning)
    error in speclite setup command: use_2to3 is invalid.
    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/83/ae/c940436d23883d4e45bfa79fca5e7cc084fd07b2722e407cfcbf6ddf47f2/speclite-0.13.tar.gz#sha256=cf32e5e7a060690fb9eecb071bbdce61854f04fe25e7fd312346a82446c56111 (from https://pypi.org/simple/speclite/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

[rrjbca]

Looks to be related to setuptools 58.0.2 which was released yesterday, specifically this pull request. However I still can't track down where the option use_2to3 is being set.

[ntessore]

It's set here in speclite==0.13.