python: bump mitogen from 0.3.12 to 0.3.13
Bumps [mitogen](https://github.com/mitogen-hq/mitogen) from 0.3.12 to 0.3.13.
- [Release notes](https://github.com/mitogen-hq/mitogen/releases)
- [Changelog](https://github.com/mitogen-hq/mitogen/blob/master/docs/changelog.rst)
- [Commits](https://github.com/mitogen-hq/mitogen/compare/v0.3.12...v0.3.13)
---
updated-dependencies:
- dependency-name: mitogen
dependency-type: direct:production
update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>