pymongoarrow has two files with the ".pyi" extension which is a mistake:

• https://github.com/mongodb-labs/mongo-arrow/blob/1998d74/bindings/python/pymongoarrow/bson.pyi
• https://github.com/mongodb-labs/mongo-arrow/blob/1998d74/bindings/python/pymongoarrow/builders.pyi

The ".pyi" is the extension for stub files. ".pxi" is the extension we're supposed to be using for Cython include files: https://cython.readthedocs.io/en/latest/src/userguide/language_basics.html#the-include-statement-and-include-files

Note that the docs also suggest to "Use Sharing Declarations Between Cython Modules instead" of including .pyi files but we only include these files in one place so I don't think there's much benefit either way. Honestly these files are so small the best option is probably to combine them directly into lib.pyx.