Binary package contains no Python dependencies, how to convince ‘dh_python3’ to detect them?

[Ben Finney <bignose@debian.org>]

Howdy all,

The Debian package ‘gandi-cli’ is building with none of the Python
library dependencies detected, so the “Depends” field contains none of
the required Python library packages. Why isn't ‘dh_python3’ reading the
dependencies from ‘requires.txt’?

When I install the upstream source, it creates the ‘requires.txt’ file
correctly. That file contains PyPI distribution names that should
already map to the correct Debian packages:

=====
IPy
click>=3.1
pyyaml
requests
setuptools
…
=====

Why is the ‘dh_python3’ run ignoring those, and expanding
“${python3:Depends}” to the empty set? How can I convince ‘dh_python3’
to correctly read the generated ‘requires.txt’?

-- 
 \       “Pinky, are you pondering what I'm pondering?” “Well, I think |
  `\       so, Brain, but ‘apply North Pole’ to what?” —_Pinky and The |
_o__)                                                           Brain_ |
Ben Finney