Enforce target and interpreter `requires-python` versions (#532)
## Summary
This PR modifies the behavior of our `--python-version` override in two
ways:
1. First, we always use the "real" interpreter in the source
distribution builder. I think this is correct. We don't need to use the
fake markers for recursive builds, because all we care about is the
top-level resolution, and we already assume that a single source
distribution will always return the same metadata regardless of its
build environment.
2. Second, we require that source distributions are compatible with
_both_ the "real" interpreter version and the marker environment. This
ensures that we don't try to build source distributions that are
compatible with our interpreter, but incompatible with the target
version.
Closes https://github.com/astral-sh/puffin/issues/407.
