[Security] Bump marshmallow from 2.13.5 to 2.15.1

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 2.13.5 to 2.15.1. **This update includes a security fix.**
- [Release notes](https://github.com/marshmallow-code/marshmallow/releases)
- [Changelog](https://github.com/marshmallow-code/marshmallow/blob/dev/CHANGELOG.rst)
- [Commits](marshmallow-code/marshmallow@2.13.5...2.15.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -22,7 +22,7 @@ itsdangerous==0.24
 Jinja2==2.8
 Mako==1.0.6
 MarkupSafe==0.23
-marshmallow==2.13.5
+marshmallow==2.15.1
 marshmallow-sqlalchemy==0.13.1
 nose==1.3.7
 packaging==16.8