build(deps-dev): update mypy requirement from ^0.761 to ^0.770

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@v0.761...v0.770)

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

Author: dependabot-preview[bot]

pyproject.toml

@@ -61,7 +61,7 @@ flake8 = "^3.6"
 pytest-cov = "^2.6"
 pytest-mock = "^2.0"
 codecov = "^2.0"
-mypy = "^0.761"
+mypy = "^0.770"
 mkdocs = "^1.0"
 mkdocs-material = "^4.1"
 isort = "^4.3.21"