This is a proactive PR to avoid breaking docs builds when Setuptools 81 starts to be used in the RTD builds process.

The sphnixcontrib-seqdiag Python package is not compatible with Setuptools 81 or later due to use of pkg_resources: https://setuptools.pypa.io/en/latest/pkg_resources.html. Warning in RTD build due to use of deprecated pkg_resources:

/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/66783/lib/python3.9/site-packages/sphinxcontrib/seqdiag.py:19: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.

Setuptools 81 release should be imminent, with the Python deprecation warning stating pkg_resources "removal as early as 2025-11-30".

Seqdiag seems to be unmaintained with the latest update at Pypi in the year 2021 and also no updates since in the git repo.

There are no seqdiag directives left in the docs after last seqdiags were removed in PR #52308.

Two other options would exist for fixing the situation but this seems to be the suitable one:

• Pin setuptools<81: this could have adverse effects later if we'd be stuck forever in an old Setuptools version.
• Fix seqdiag code: this does not seem to be worthwhile when the functionality of the package is not even used.

This PR also removes following Python packages that were dependencies of only sphinxcontrib-seqdiag:

• blockdiag: discussion about this exists in https://docs.ceph.com/en/latest/dev/documenting/#blockdiag and I believe it is written in this way because it is simply out of date. In fact the whole blockdiag functionality was never configured in Sphinx; only the seqdiag (which seems to be a part of the "blockdiag suite") was configured.
• seqdiag
• Pillow
• webcolors

Collecting blockdiag>=1.5.0 (from sphinxcontrib-seqdiag->-r admin/doc-requirements.txt (line 18))
  Downloading blockdiag-3.0.0-py3-none-any.whl.metadata (4.6 kB)
Collecting seqdiag>=0.9.3 (from sphinxcontrib-seqdiag->-r admin/doc-requirements.txt (line 18))
  Downloading seqdiag-3.0.0-py3-none-any.whl.metadata (3.4 kB)
Collecting Pillow>3.0 (from blockdiag>=1.5.0->sphinxcontrib-seqdiag->-r admin/doc-requirements.txt (line 18))
  Downloading pillow-11.3.0-cp39-cp39-manylinux_2_27_x86_64.manylinux_2_28_x86_64.whl.metadata (9.0 kB)
Collecting webcolors (from blockdiag>=1.5.0->sphinxcontrib-seqdiag->-r admin/doc-requirements.txt (line 18))
  Downloading webcolors-24.11.1-py3-none-any.whl.metadata (2.2 kB)

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