Allow pathlib in types

I did this:

```py
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.