Changed fs::path to be bound to os.PathLike|str.

Strings are implicitly convertible to `fs::path`, but pybind11 just uses
`os.PathLike` and not `os.PathLike | str` as type annotation.
Python does not support implicit conversions for annotations, so this
corrects the annotations to be explicit.

cpp/pybind/pybind_filesystem.h

@@ -99,7 +99,7 @@ struct path_caster {
         return true;
     }
 
-    PYBIND11_TYPE_CASTER(T, const_name("os.PathLike"));
+    PYBIND11_TYPE_CASTER(T, _("Union[os.PathLike, str]"));
 };
 
 template <>