Remove python-pycountry from build-deps to simplify Python 2 removal

python-formencode is the last rdep for python-pycountry. The package does not indicate any runtime dependency on pycountry and pycountry is only needed at build-time for testing.

This MR disables the tests that will fail in the absence of pycountry, plus also deals with a few additional packaging bits and pieces that range from minor to important, as documented in the changelog entry.

DPMT is in "Uploaders" not "Maintainer", so an explicit endorsement from the regular maintainers is desired.