Update flit requirement from ~=3.3 to ~=3.4
Updates the requirements on [flit](https://github.com/takluyver/flit) to permit the latest version.
- [Release notes](https://github.com/takluyver/flit/releases)
- [Changelog](https://github.com/takluyver/flit/blob/master/doc/history.rst)
- [Commits](https://github.com/takluyver/flit/compare/3.3.0...3.4.0)
---
updated-dependencies:
- dependency-name: flit
dependency-type: direct:production
...
Signed-off-by: dependabot[bot] <support@github.com>