Move isort config to pyproject.toml and add black profile

[zero323]

This PR:

• Removes profile=black option for isort from CI and adds it to respective configuration
• Moves isort config into pyproject.toml and drops setup.cfg.

Reasons

Currently isort's --profile=black is used only in CI, which means that you have to remember do the same locally (which I personally forgot). This can lead to situations where isort doesn't detect any problems locally (if profile is accidentally committed) but fails in CI.

Additionally, isort recommends pyproject.toml for storing configuration and setup.cfg is not used for anything else (other dev tool configs could be migrated as well).

[zero323]

That's strange (and seems to be already an issue on master, when mypy==0.920 is used :(

pytest_mypy_plugins/collect.py:79: error: Item "None" of "Union[local, Any, None]" has no attribute "read_text"
pytest_mypy_plugins/item.py:350: error: Incompatible return value type (got "Tuple[Union[local, Any, None], None, str]", expected "Tuple[Union[local, str], Optional[int], str]")
Found 2 errors in 2 files (checked 8 source files)
Error: Process completed with exit code 1.

[sobolevn]

Thanks!