This package requires at least Python 3.7 (it will possibly work with some
extra packages from PyPI that have been included in Python since Python 3.7,
but these aren't currently packaged in Debian).

Since Python 3.7 isn't default in Debian yet, this required the following to
work correctly:

1) Build-Depends on python3.7 as an alternative to python3 (>= ~3.7~) which
isn't yet available.

2) Dropping of ${python3:Depends} from binary package dependencies, since
dh-python insists on adding an arbitrary python3:any (>= 3.7~) that cannot yet
be satisfied, and there doesn't seem to be any option to turn this off.
Instead, we must add the necessary dependencies (none) by hand.

3) Manual specification of PYBUILD_INTERPRETERS and PYBUILD_VERSIONS in
debian/rules to be 3.7 (only).

4) Lintian overrides for both the source and the binary because the lintian
checks don't seem to accept "python3 (>= 3.7~) | python3.7", neither for
Build-Depends nor for Depends, even though they are sufficient in this case.

These four changes can be dropped once the python3 package in Debian reaches
(>= 3.7).
