core (memory/crypto): replaced PyCrypto by cryptography:

`PyCrypto` is unmaintained for years but was used in SàT for password hashing. This patch
fixes that by replacing `PyCrypto` by the reference `cryptography` module which is well
maintained.

The behaviour stays the same (except that previously async `hash`, `encrypt` and `decrypt`
methods are now synchronous, as they are quick and using a deferToThread may actually be
more resource intensive than using blocking methods).

It is planed to improve `memory.crypto` by using more up-to-date cryptography/hashing
algorithms in the future.

PyCrypto is no more a dependency of SàT

setup.py

@@ -35,7 +35,6 @@ install_requires = [
     'netifaces',
     'pillow',
     'progressbar2',
-    'pycrypto >= 2.6.1',
     'cryptography',
     'pygments',
     'pygobject',