✨ feat(spec): accept build suffixes on version specifiers - #176

Since 1.6.6, >=3.13t is looked up as an executable named >=3.13t (#175), so tools such as nox that let users write python=">=3.13t" find nothing. Before 1.6.6 the spec parsed, but discovery raised AssertionError on the unreadable 3.13t item, and under -O it dropped the t and could pick a GIL build. ✨ Once this merges, >=3.13t asks for a free-threaded 3.13 or newer, and the other build suffixes work on ranges too: cpython>=3.11,<3.14-64 asks for a 64-bit CPython in that range.

Plain specs (python3.13td-64-arm64) and specifiers now share one grammar for the trailing build modifiers: t, d/-dbg/-debug, -32/-64 and -<machine>. On a specifier they follow the last item and apply to the whole set, the same place uv puts the variant in a --python request. Without t a specifier still accepts any build, so existing ranges match the same interpreters as before.

⚠️ Behavior change

A spec that starts with a comparison operator but doesn't parse, such as >=3.13., >=3.x, >=3.12 <4 (missing comma) or >=3.13t,<3.15 (a flag on an inner item), now raises ValueError naming the spec. In 1.6.6 and 1.6.7 it became a literal executable name, which matched nothing and let a key list move on to the next entry. Callers that pass user input straight to get_interpreter or PythonSpec.from_string_spec see the error instead of a silent miss; wntrblm/nox#1217 handles unparsable ranges by checking for that literal-path result, so it needs to catch the ValueError instead.