Bump marshmallow from 2.12.2 to 2.15.1

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 2.12.2 to 2.15.1.
- [Release notes](https://github.com/marshmallow-code/marshmallow/releases)
- [Changelog](https://github.com/marshmallow-code/marshmallow/blob/dev/CHANGELOG.rst)
- [Commits](marshmallow-code/marshmallow@2.12.2...2.15.1)

Signed-off-by: dependabot[bot] <[email protected]>