Update RSA-PSS docs for Python 3.

Update message to byte string.

Update RSA.import_key() to read DER file in binary.

Doc/src/signature/pkcs1_pss.rst

@@ -12,10 +12,9 @@ The following example shows how the sender can use its own *private* key
     >>> from Crypto.Signature import pss
     >>> from Crypto.Hash import SHA256
     >>> from Crypto.PublicKey import RSA
-    >>> from Crypto import Random
     >>>
-    >>> message = 'To be signed'
-    >>> key = RSA.import_key(open('privkey.der').read())
+    >>> message = b'To be signed'
+    >>> key = RSA.import_key(open('privkey.der', 'rb').read())
     >>> h = SHA256.new(message)
     >>> signature = pss.new(key).sign(h)