mypy: upgrade to 0.812

[lionello]

nix-shell failed with Nixpkgs from unstable channel (nixpkgs-21.11pre317460.a54d2e72e28) because of an error with mypy v0.770:

Processing /private/tmp/nix-build-python3.9-mypy-0.770.drv-0/mypy-0.770
  Created temporary directory: /private/tmp/nix-build-python3.9-mypy-0.770.drv-0/pip-req-build-crybcke8
  Added file:///private/tmp/nix-build-python3.9-mypy-0.770.drv-0/mypy-0.770 to build tracker '/private/tmp/nix-build-python3.9-mypy-0.770.drv-0/pip-req-tracker-2damr_g5'
    Running setup.py (path:/private/tmp/nix-build-python3.9-mypy-0.770.drv-0/pip-req-build-crybcke8/setup.py) egg_info for package from file:///private/tmp/nix-build-python3.9-mypy-0.770.drv-0/mypy-0.770
    Created temporary directory: /private/tmp/nix-build-python3.9-mypy-0.770.drv-0/pip-pip-egg-info-a_g10rel
    Running command python setup.py egg_info
    mypy/api.py:52: error: syntax error in type comment
    mypy/api.py:66: error: syntax error in type comment
    mypy/api.py:73: error: syntax error in type comment
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

This PR relaxes the version of mypy, since "^0.770" practically pins it to v0.770 and won't allow any other version. (Poetry treats any 0.x as a breaking change so will never bump a minor version below v1.)

Similarly, v0.910 failed to build because of an error similar to https://github.com/nix-community/poetry2nix/issues/323 so adding constraint to <v0.900.

[lionello]

I don't know why the CI fails with invalid Git revision 'master'. Don't know how that's related to mypy :( @adisbladis Any idea?