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>