Correct type annotations for original_path

[dhdaines]

Fixes: #1180

As mentioned in that issue, the type annotation for the original_path attribute of LTCurve doesn't correspond to the actual data structure. This is maybe a bad thing for people who use type checkers 😉

This makes it very type-safe, with actual literals for the operators. Unfortunately mypy doesn't yet know how to narrow this properly, but it may be useful in the future.

How Has This Been Tested?

By running mypy on the code.

Checklist

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