Add all the email __all__s (#7304)

stdlib/email/base64mime.pyi

@@ -1,3 +1,5 @@
+__all__ = ["body_decode", "body_encode", "decode", "decodestring", "header_encode", "header_length"]
+
 def header_length(bytearray: str | bytes) -> int: ...
 def header_encode(header_bytes: str | bytes, charset: str = ...) -> str: ...
 def body_encode(s: bytes, maxlinelen: int = ..., eol: str = ...) -> str: ...