Bump mypy from 0.750 to 0.761

Bumps [mypy](https://github.com/python/mypy) from 0.750 to 0.761.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](https://github.com/python/mypy/compare/v0.750...v0.761)

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

requirements-dev.txt

@@ -7,7 +7,7 @@ coveralls==1.9.2
 flake8==3.7.9
 flake8-type-annotations==0.1.0
 flake8-tidy-imports==3.1.0
-mypy==0.750
+mypy==0.761
 pytest==5.3.2
 pytest-asyncio==0.10.0
 pytest-cov==2.8.1