c/fields: Reduce severity a python b-d tag to minor

Reduce to severity of build-depends-on-python-dev-with-no-arch-any to
minor (from normal).

Signed-off-by: Niels Thykier <niels@thykier.net>
d/changelog: Update the tag summary

Signed-off-by: Niels Thykier <niels@thykier.net>