Make groups-related functions work with str instead of bytes

(get_user_groups/get_group_members/group_required)

This was undone accidentally when removing support for Python2 in
https://github.com/admiralobvious/flask-simpleldap/commit/b2c0d10332d27db8b349753f89dceb7db520b7f2#diff-df961969fdc0fcec2f458e0fbb16eef9L238

Closes #63, #64.