Update mypy requirement from ~=0.930 to ~=0.931 (#1078)

Updates the requirements on [mypy](https://github.com/python/mypy) to permit the latest version.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.930...v0.931)

---
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>

dev_requirements.txt

@@ -10,7 +10,7 @@ pytest-xdist[psutil]~=2.5.0
 pytest-cov~=3.0.0
 flake8~=4.0.1
 black~=19.10b0
-mypy~=0.930
+mypy~=0.931
 
 # Documentation and examples.
 sphinxcontrib-bibtex~=2.4.1