Bump mypy from 0.761 to 0.770 (#28)

Bumps [mypy](https://github.com/python/mypy) from 0.761 to 0.770.
- [Release notes](https://github.com/python/mypy/releases)
- [Commits](python/mypy@v0.761...v0.770)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements.txt

@@ -1,7 +1,7 @@
 entrypoints==0.3
 flake8==3.7.9
 mccabe==0.6.1
-mypy==0.761
+mypy==0.770
 pycodestyle==2.5.0
 pyflakes==2.1.1
 typed-ast==1.4.1