Permit configuration via a stand-alone TOML file (`mypy.toml`)

[AA-Turner]

Feature

In #5205, MyPy gained support for declaring configuration in the TOML format, but only in pyproject.toml. I propose supporting configuration via a stand-alone TOML document, such as mypy.toml or .mypy.toml.

Pitch

This parallels, and perhaps would eventually replace, configuration via mypy.ini. This is most useful when checking non-project code, such as in Python (python/cpython#129708), where we have several distinct mypy.ini files to support type-checking different directories of a larger project. It may be useful for mono-repo layouts, where again the use of pyproject.toml is undesirable. Finally, this would be useful to keep tool configuration to independent files — in the Sphinx project, there are several hundred lines of mypy configuration in pyproject.toml, dominating the content of the file.

A

[Avasam]

The .ini format also has its limitations. Like not supporting comments at the end of a line and a weird exclude.

Additional benefits includes: a single toml formatter (rather than needing an ini formatter just for this config). And IntelliSense could be provided through a schema.