python: Fix wrong use of write.

* lang/python/helpers.c (pyPassphraseCb): Use gpgme_io_writen.
(_gpg_interact_cb): Ditto.
* lang/python/helpers.h (write) [W32]: Remove bad write macro.

Using write(2) without checking for EINTR is a not a good idea.
Futher gpgme_io_writen is the correct way to send data from a
callback. This also fixed the wrong use of a simple macro for
Windows.

• GnuPG-bug-id: T6501