Update mypy requirement from >=0.991,<1.5 to >=0.991,<1.8

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@v0.991...v1.7.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -51,5 +51,5 @@ isort = "^5.6.4"
 pytest-cov = "^4.0.0"
 Sphinx = "^5.0.2"
 sphinx-rtd-theme = "^2.0.0"
-mypy = ">=0.991,<1.5"
+mypy = ">=0.991,<1.8"
 pylint = '^3.0'