validator: fix ASCII password check to verify if it can be *encoded* in ascii

In Python 2, unicode strings have a .decode method (which really doesn't make
sense). Python 3 has more strict typing by design, and unicode strings don't
have a .decode method.

A Unicode string "is ASCII" if it can be encoded as ASCII. The check should
thus *encode* to ASCII - not decode.

kallithea/model/validators.py

@@ -235,7 +235,7 b' def ValidPassword():'
 
         def _validate_python(self, value, state):
             try:
-                (value or '').decode('ascii')
+                (value or '').encode('ascii')
             except UnicodeError:
                 msg = self.message('invalid_password', state)
                 raise formencode.Invalid(msg, value, state,)