Changelog:
==========
- Add __len__ implementation to missing so that it can be used with validate.Length <marshmallow.validate.Length>
- Drop support for Python 3.9
- Test against Python 3.14.
Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
