black compatibility: ignore lines_after_imports for .pyi typing stubs

[XuehaiPan]

isort and black are two famous quality control tools. And usually, they are used together. Black Compatibility

The user may set custom lines_after_imports in their configuration file (e.g. pyproject.toml):

[tool.black]
safe = true
line-length = 88
skip-string-normalization = true
target-version = ["py37", "py38", "py39", "py310"]

[tool.isort]
atomic = true
profile = "black"
indent = 4
line_length = 100
lines_after_imports = 2
multi_line_output = 3

For .py files, the maximum empty lines for black are 2 and everything works fine. However, for .pyi stubs files, black only allows a single blank line at the top level, while isort adds an extra blank line when setting lines_after_imports = 2.

https://github.com/psf/black/blob/b60b85b234d6a575f636d0a125478115f993c90c/src/black/lines.py#L485-L487

For .pyi files, if the user runs black first and then isort, there will be 2 empty lines after imports. If the user runs isort before black, there will be only 1 empty line after imports. Both black and isort will modify the .pyi files. And command:

isort --check *.pyi && black --check *.pyi

always fails if set lines_after_imports = 2.

The user have to add isort: off at the top of all .pyi files.