Lintian tag information: alternatively-build-depends-on-python-sphinx-and-python3-sphinx (type: warning)

Description (from lintian-explain-tags)

This package alternatively Build-Depends on the Python 2 or Python 3
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".

Please replace the alternative with a single build dependency on
python3-sphinx.

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

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