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

sat/plugins/plugin_sec_otr.py

@@ -248,12 +248,8 @@ class Account(potr.context.Account):
         if self.privkey is None:
             raise exceptions.InternalError(_("Save is called but privkey is None !"))
         priv_key = hexlify(self.privkey.serializePrivateKey())
-        d = self.host.memory.encryptValue(priv_key, self.client.profile)
-
-        def save_encrypted_key(encrypted_priv_key):
-            self.client._otr_data[PRIVATE_KEY] = encrypted_priv_key
-
-        d.addCallback(save_encrypted_key)
+        encrypted_priv_key = self.host.memory.encryptValue(priv_key, self.client.profile)
+        self.client._otr_data[PRIVATE_KEY] = encrypted_priv_key
 
     def loadTrusts(self):
         trust_data = self.client._otr_data.get("trust", {})
@@ -377,7 +373,7 @@ class OTR(object):
         yield client._otr_data.load()
         encrypted_priv_key = client._otr_data.get(PRIVATE_KEY, None)
         if encrypted_priv_key is not None:
-            priv_key = yield self.host.memory.decryptValue(
+            priv_key = self.host.memory.decryptValue(
                 encrypted_priv_key, client.profile
             )
             ctxMng.account.privkey = potr.crypt.PK.parsePrivateKey(