Fix `.as_uri()` and `.absolute()` implementations for `WorkspacePath`

[asnare]

This PR is a rebase that replaces #126 and fixes some things that the integration tests expect but weren't covered by unit tests:

• .as_uri(), which internally depends on the __bytes__() dunder method.
• .absolute(), which is supported in the trivial (no-op) case.

Both underlying implementations depend on PathLib internals that changed across python versions.

[github-actions[bot]]

✅ 18/18 passed, 2 skipped, 35s total

_{Running from acceptance #181}