Bump marshmallow from 0.7.0 to 2.15.1 in /blog/server
Bump marshmallow from 0.7.0 to 2.15.1 in /blog/server

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 0.7.0 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](https://github.com/marshmallow-code/marshmallow/compare/0.7.0...2.15.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
amponce/simpleblog,amponce/simpleblog,amponce/simpleblog