fix: format_python_constraint IndexError on unions outside PYTHON_VERSION

[otiscuilei]

format_python_constraint walks a hardcoded PYTHON_VERSION list (currently through 3.14) and then takes accepted[0] as the lower bound. A VersionUnion that matches none of those series raises IndexError while writing wheel Requires-Python.

That path is used for legacy [tool.poetry.dependencies] python = "…" (PEP 621 requires-python bypasses the helper). Adjacent ranges such as ~3.15 || ~3.16 merge into a single VersionRange and are fine. Non-adjacent unions do not merge and crash:

from pathlib import Path
from poetry.core.masonry.metadata import Metadata
from poetry.core.packages.project_package import ProjectPackage

pkg = ProjectPackage("foo", "1.0")
pkg.root_dir = Path(".")
pkg.python_versions = "~3.15 || ~3.17"  # skip 3.16; also "4.0 || 4.1"
Metadata.from_package(pkg)
# IndexError: list index out of range

^3.15 / >=3.15 / ~3.15 are ranges and already work. Falling back to str(union) would emit poetry || syntax, which is not valid PEP 440.

When nothing in PYTHON_VERSION matched, take the union's own lower bound and keep excluding the known series — the same approximation the helper already uses for in-list unions (3.8 || 3.10 also allows later 3.x not in the list). This does not add 3.15 to PYTHON_VERSION (see #961).

• Added tests for changed code.
• Updated documentation for changed code. — n/a

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Needs a human reviewer. If the formatting is wrong, published package metadata could allow installation on an unsupported Python version or reject a supported one, and reverting would not change metadata already released. The impact is bounded to affected package artifacts and can be repaired with a subsequent release.

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