Make the mandatory metadata warning generic

This should close #95.
8 jobs from master in 4 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Linting
passed #87835
bandit

00:00:52

passed #87838
mypy

00:00:31

passed #87837
pyflakes

00:00:35

passed #87836
pylint

00:01:13

 
  Test
passed #87842
whitewhale
tests:archlinux

00:02:19

passed #87839
tests:debian

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98.0%
passed #87840
whitewhale
tests:debian_with_bubblewrap

00:03:31

100.0%
passed #87841
whitewhale
tests:fedora

00:03:11