Fixes #182.

Reasoning for the change was:

I think we are just lying to ourselves when we keep PGP module in the
repository.

1. It is not used. Over a year, there is ``DeprecationWarning`` asking
   anybody who uses the module to contact me. Nobody ever did. Even the
   previous maintainer of M2Crypto [did not know how to use
   it](https://stackoverflow.com/a/1042139/164233), and that was 2009.
   Whole Stack Overflow is full of vague comments like “hopefully
   M2Crypto.PGP will help you”, but I have not found on DuckDuckGo one
   example of use of M2Crypto.PGP

2. The main point of M2Crypto is to be Python bindings for OpenSSL. And
   truly most of the other M2Crypto code is just a tiny wrapper around
   C functions. That is not so with M2Crypto.PGP. It is basically
   reimplementation of PGP keys manipulation functions with Python with
   rather large set of Python objects and complicated computations. There
   are very few calls to any C code at all and most work is done...