[Authlib] Add default for generate_token(chars) (#15787)

Author: mrhx01

stubs/Authlib/authlib/common/security.pyi

@@ -2,5 +2,5 @@ from typing import Final
 
 UNICODE_ASCII_CHARACTER_SET: Final[str]
 
-def generate_token(length: int = 30, chars: str = ...) -> str: ...
+def generate_token(length: int = 30, chars: str = UNICODE_ASCII_CHARACTER_SET) -> str: ...  # noqa: Y011
 def is_secure_transport(uri: str) -> bool: ...