update load() to use Pathlib object open

[klinden]

The load() method already accepts either a string or Pathlib object. Instead of converting the Pathlib object to a string and using the global open() function, convert strings to Pathlib objects and use Path.open(). This allows for passing and correctly handling objects that extend Path.

[tomMoral]

Hello! Thanks for the PR, the change seems useful.

It is failing part of the CI because it looks like it breaks the file's tracking with the resource manager. the _unpickle function expects a str filename but is provided with a Path. We need to adress this point before merging the PR.