Paroxython

- Actually delete labels scheduled for deletion even if they are derived. For instance, previously, the following double scheduling was only honored for adding `pure_function`, not for deleting `impure_function`:

python
def gray(n): paroxython: -impure_function:gray... +pure_function:gray...
if n <= 0:
return [""]
else:
previous = gray(n - 1)
return prefix_all(previous, "0") + prefix_all(reversed(previous), "1") paroxython: ...impure_function:gray ...pure_function:gray


- Fix `paroxython collect` default --skip argument.

- Allow alternative results when testing `spec.md` examples (depending under which Python version Paroxython runs).
- Extend `no_operation` to Ellipsis syntax.
- Add `assignment_expression` feature.
- Correct the span of decorated functions and classes.
- Make `flatten_ast.py` to work with both `typed-ast` and `ast` library.
- Add a helper to compare the results of their respective parser.
- Upgrade the development environment to Python 3.8.