Add email.base64mime stub (#6659)

stdlib/email/base64mime.pyi

@@ -0,0 +1,7 @@
+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: ...
+def decode(string: str | bytes) -> bytes: ...
+
+body_decode = decode
+decodestring = decode