Add __all__ to modules beginning with 'm' (#7330)

stdlib/mailcap.pyi

@@ -2,6 +2,8 @@ from typing import Mapping, Sequence, Union
 
 _Cap = dict[str, Union[str, int]]
 
+__all__ = ["getcaps", "findmatch"]
+
 def findmatch(
     caps: Mapping[str, list[_Cap]], MIMEtype: str, key: str = ..., filename: str = ..., plist: Sequence[str] = ...
 ) -> tuple[str | None, _Cap | None]: ...