text: Title: Make `TestSuite.source` attribute `pathlib.Path` instance
  Body: Currently `TestSuite.source` is a string, but [pathlib.Path](https://docs.python.org/3/library/pathlib.html) would be more convenient for users of this API. More importantly, processing file system paths during parsing is easier by using `Path` instances instead of strings. It wouldn't make sense to convert them to strings when the `TestSuite` is created.
  
  This is potentially backwards incompatible if someone has used `source` as a string like `source.endswith('.robot')`. There generally shouldn't be such needs and converting the source to a string is easy as well. Importantly, manipulating `source` using `os.path` functions will work the same way as earlier. I'll mark this `backwards incompatible` so that it gets mentioned in the release notes, but I don't consider this so severe that it needed to wait for a major release.