Fix TypeError in call to bcrypt.hashpw

- At the very least, this TypeError caused logins to fail on my own
  running instance of Synapse, and the simple (explicit) UTF-8
  conversion resolved login errors for me.

Signed-off-by: Salvatore LaMendola <salvatore.lamendola@gmail.com>

synapse/handlers/auth.py

@@ -626,6 +626,6 @@ class AuthHandler(BaseHandler):
             Whether self.hash(password) == stored_hash (bool).
         """
         if stored_hash:
-            return bcrypt.hashpw(password, stored_hash) == stored_hash
+            return bcrypt.hashpw(password, stored_hash.encode('utf-8')) == stored_hash
         else:
             return False