gh-90890: New methods to access mailbox.Maildir message info and flags (#103905)

New methods to access mailbox.Maildir message info and flags:
get_info, set_info, get_flags, set_flags, add_flag, remove_flag.

These methods speed up accessing a message's info and/or flags and are
useful when it is not necessary to access the message's contents,
as when iterating over a Maildir to find messages with specific flags.

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* Add more str type checking
* modernize to f-strings instead of %

Co-authored-by: Gregory P. Smith <greg@krypto.org>