python: Rename exported functions.
Avoid the name pygpgme, as this is the name of another popular Python
binding for GPGME.
This commit renames all functions that are exported to the Python
world.
- lang/python/helpers.c: Rename all exported functions.
- lang/python/helpers.h: Likewise.
- lang/python/pyme/core.py: Likewise.
- Signed-off-by: Justus Winter <justus@g10code.com>