Always return a `str` from `literal_name`

[pietermarsman]

Pull request

This is what happened in practice for correct input (e.g. PSLiteral). But now it also always returns a str when the object is not a PSLiteral.

This fixes #977 because it makes the literal name object hashable. This sample PDF will now crash with a PDFSyntaxError. That's good.

How Has This Been Tested?

With the PDF from #977. And with nox.

Checklist

• [x] I have read CONTRIBUTING.md.
• [x] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x] I have tested that this fix is effective or that this feature works.
• [x] I have added docstrings to newly created methods and classes.
• [x] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.