Bug#944774: Support python 3.8

[Matthias Klose]

Control: severity -1 sersious
Control: tags -1 + patch

please see the patch debian/patches/0007-Python-Versions.patch in
https://patches.ubuntu.com/g/gpgme1.0/gpgme1.0_1.12.0-6ubuntu3.patch

to build against all supported Python3 versions.