Support for black's --pyi flag?

[AlexWaygood]

Description

Hi! Thanks for maintaining blacken-docs -- it's a great project!

Black supports a --pyi flag; if you pass this flag, it formats all code as if it were a .pyi stub file. (In practice, this results in more concise formatting with fewer blank lines.) I'm working on a project where we'd ideally like our Python examples in Markdown files formatted quite concisely, as if they were stubs. However, there doesn't seem to be any way currently to do this: black-as-invoked-by-blacken-docs doesn't take notice of black configuration options in a [tool.black] pyproject.toml table (I think because you're invoking Black as a library rather than as a subprocess), and blacken-docs has no flag or config option to format docs examples as stubs.

Would you be open to adding a --pyi flag to blacken-docs, that would simply be passed directly on to Black? (I'd be very happy to submit a PR, if so!)

[adamchainz]

Sure, happy to chuck it on the list of options passed through to black.