Update mypy requirement from ~=1.7.0 to >=1.7,<1.9 (#526)

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@v1.7.0...v1.8.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

setup.py

@@ -29,7 +29,7 @@ def find_stub_files(name: str) -> List[str]:
 
 # Keep compatible-mypy major.minor version pinned to what we use in CI (requirements.txt)
 extras_require = {
-    "compatible-mypy": ["mypy~=1.7.0", "django-stubs[compatible-mypy]"],
+    "compatible-mypy": ["mypy>=1.7,<1.9", "django-stubs[compatible-mypy]"],
     "coreapi": ["coreapi>=2.0.0"],
     "markdown": ["types-Markdown>=0.1.5"],
 }