Remove unused "NULL" from __all__ in __init__.py (PyMySQL#369)

Since the symbol "NULL" is removed in 819688b, it should be also
removed from `__all__`. Fix issue PyMySQL#368.

Author: ckclark

MySQLdb/__init__.py

@@ -89,7 +89,7 @@ def Connect(*args, **kwargs):
     'Date', 'Time', 'Timestamp', 'DateFromTicks', 'TimeFromTicks',
     'TimestampFromTicks', 'DataError', 'DatabaseError', 'Error',
     'FIELD_TYPE', 'IntegrityError', 'InterfaceError', 'InternalError',
-    'MySQLError', 'NULL', 'NUMBER', 'NotSupportedError', 'DBAPISet',
+    'MySQLError', 'NUMBER', 'NotSupportedError', 'DBAPISet',
     'OperationalError', 'ProgrammingError', 'ROWID', 'STRING', 'TIME',
     'TIMESTAMP', 'Warning', 'apilevel', 'connect', 'connections',
     'constants', 'converters', 'cursors', 'debug', 'escape',