build(deps-dev): bump mypy from 1.7.0 to 1.8.0 in /backend/requirements

Bumps [mypy](https://github.com/python/mypy) from 1.7.0 to 1.8.0.
- [Changelog](https://github.com/python/mypy/blob/master/CHANGELOG.md)
- [Commits](python/mypy@v1.7.0...v1.8.0)

---
updated-dependencies:
- dependency-name: mypy
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

backend/requirements/development.txt

@@ -6,7 +6,7 @@ watchfiles==0.21.0  # https://github.com/samuelcolvin/watchfiles
 
 # Testing
 # ------------------------------------------------------------------------------
-mypy==1.7.0  # https://github.com/python/mypy
+mypy==1.8.0  # https://github.com/python/mypy
 django-stubs==4.2.4  # https://github.com/typeddjango/django-stubs
 pytest-sugar==0.9.7  # https://github.com/Frozenball/pytest-sugar