Require Python >= 3.3

When porting to Python3, I originally kept it working with Python2 as
well. But I didn’t test with Python2 for a while and the current code
would not work anymore with Python2.  It doesn’t seem to make sense
to invest effort to keep Python2 compatibility. Better require Python >= 3.3.
Escape % and _ if they are not intended as wildcards

Resolves: rhbz#1123981 - With the new "rusle" table in ibus-table-cyrillic, typing '%' shows ':' in preëdit instead of committing it directly
See: https://bugzilla.redhat.com/show_bug.cgi?id=1123981
Release v1.8.6