Update mypy requirement from <=1.8.0 to <=1.9.0 in /server

Updates the requirements on [mypy](https://github.com/python/mypy) to permit the latest version.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](python/mypy@v0.1.0...1.9.0)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:development
...

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

server/dev-requirements.txt

@@ -4,7 +4,7 @@ pep8
 pytest
 flake8
 nose
-mypy<=1.8.0
+mypy<=1.9.0
 httmock==1.4.0
 wooper==0.4.4
 requests_mock