Explicit the UTF-8 encoding also when installing using Python 3

If the locale isn't UTF-8, or for some reason Python doesn't pick up on
it, it will try to decode using ASCII, which will of course cause
mayhem, crash and despair.

This patch will be shipped with the Debian package 1.1.0-1
Merge pull request #25 from laarmen/fixup_setup_py3k

Explicit the UTF-8 encoding also when installing using Python 3