installed-tests: Downgrade python2 from Depends to Suggests

Test coverage will be a bit better if it is installed, but this doesn't
need to block the eventual removal of python2 from Debian.

The test in question relies on implementation details of Python 2,
so it is not straightforward to convert to Python 3.

Closes: #936530
8 jobs for debian/master in 9 minutes and 59 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #299131
build

00:04:15

 
  Test
passed #299136
autopkgtest

00:03:24

passed #299137
blhc

00:01:29

passed #299135
lintian

00:01:46

passed #299138
piuparts

00:05:42

failed #299134
reprotest

00:04:29

passed #299133
test-build-all

00:02:23

passed #299132
test-build-any

00:03:44

 
Name Stage Failure
failed
reprotest Test
        if ( cleanup ); then :; else echo >&2 "cleanup failed with exit code $?"; fi;
fi
exit $__x
fi" failed with status 2
Uploading artifacts...
/builds/debian/flatpak-builder/debian/output/reprotest: found 6 matching files
/builds/debian/flatpak-builder/debian/output/reprotest.log: found 1 matching files

Uploading artifacts to coordinator... ok
id=299134 responseStatus=201 Created token=am4zUS6S
ERROR: Job failed: exit code 1