Allow pathlib in types

[verhovsky]

I did this:

from pathlib import Path
import frontmatter

markdown_file = Path("build") / "foo.md"
loaded = frontmatter.load(markdown_file)

What I got

Argument of type "Path" cannot be assigned to parameter "fd" of type "str | IOBase" in function

The code works, you just have to fix the types.

[merwok]

(title should say os.PathLike, not pathlib)