Missed this part when initially going through the PEP

Per PEP 639:

Path delimiters MUST be the forward slash character (/). Patterns are relative to the directory containing pyproject.toml, therefore the leading slash character MUST NOT be used.

Currently the hook is written to check relative to the current working directory. This is probably going to be equivalent in most cases, but might as well make it more correct. Because the hook should be explicitly matching pyproject.toml, we should be able to assume the parent of the file received by the hook as our base directory.