Add DeprecationWarning for auth

http2irc.py

@@ -19,6 +19,7 @@ import string
 import sys
 import time
 import toml
+import warnings
 
 
 logger = logging.getLogger('http2irc')
@@ -184,6 +185,7 @@ class Config(dict):
 				if 'auth' in map_:
 					if 'postauth' in map_:
 						raise InvalidConfig(f'auth and postauth are aliases and cannot be used together')
+					warnings.warn('auth is deprecated, use postauth instead', DeprecationWarning)
 					map_['postauth'] = map_['auth']
 					del map_['auth']
 				for k in ('postauth', 'getauth'):