The default encoding for source files is UTF-8 since Python 3, so we can
drop all encoding headers. While we are at it, just replace them with
SPDX headers.
This basically reverts commit 86b31dc12b.
It now works like this: If the use has pycountry installed, it is used.
If not, Dosage falls back to a small internal list generated from
pycountry by scripts/mklanguages.py.
This means additional work if we ever decide to translate Dosage, since
pycountry already has all the translations for language names...
This fixes#23.
