pep508: implement marker simplification/complexification
This adds new routines to `MarkerTree` for "simplifying" and
"complexifying" a tree with respect to lower and upper Python version
bounds.
In effect, "simplifying" a marker is what you do when you write it to a
lock file. Namely, since `uv.lock` includes a `requires-python` bound at
the top, one can say that it acts as a bound on the supported Python
versions. That is, it establishes a context in which one can assume that
bound is true. Therefore, the markers we write can be simplified using
this assumption.
The reverse is "complexifying" a marker, and it's what you do when you
read a marker from the lock file. Namely, once a marker is read, it can
be very difficult in code to keep the corresponding requires-python
context from the lock file. If you lose track of it and decide to
operate on the "simplified" marker, then it's trivial for that to
produce an incorrect result.
