Lintian tag information: build-depends-on-python-sphinx-only (type: warning)

Description (from lintian-explain-tags)

This package Build-Depends on the Python 2.x version of the Sphinx
documentation generator.

The 2.x series of Python is due for deprecation and will not be maintained
by upstream past 2020 and will likely be dropped after the release of
Debian "buster".

Some Python modules may need to depend on both python-sphinx and
python3-sphinx but please consider moving to only Build-Depending on the
python3-sphinx package instead.

Visibility: warning
Show-Always: no
Check: languages/python

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