Fix handling of != intersections in `requires-python` (#7897)
## Summary
The issue here is that, if you user has a `requires-python` like `>=
3.7, != 3.8.5`, this gets expanded to the following bounds:
- `[3.7, 3.8.5)`
- `(3.8.5, ...`
We then convert this to the specific `>= 3.7, < 3.8.5, > 3.8.5`. But the
commas in that expression are conjunctions... So it's impossible to
satisfy? No version is both `< 3.8.5` and `> 3.8.5`.
Instead, we now preserve the input `requires-python` and just
concatenate the terms, only using PubGrub to compute the _bounds_.
Closes https://github.com/astral-sh/uv/issues/7862.
