Allow download of Python distribution variants with newer CPU instruction sets (#9781)

Supersedes https://github.com/astral-sh/uv/pull/8517 with an alternative
approach of making all the variants available instead of replacing the
x86_64 (v1) variant with x86_64_v2.

Doesn't add automatic inference of the supported instructions, but that
should be doable per @charliermarsh's comment there. Going to do it as a
follow-up since this has been pretty time consuming.

e.g.,

```
❯ cargo run -q -- python install cpython-3.12.8-linux-x86_64_v3-gnu
Installed Python 3.12.8 in 2.72s
 + cpython-3.12.8-linux-x86_64_v3-gnu
```

Co-authored-by: j178 <10510431+j178@users.noreply.github.com>

crates/uv-python/fetch-download-metadata.py

@@ -50,7 +50,7 @@ import json
 import logging
 import os
 import re
-from dataclasses import dataclass, field
+from dataclasses import asdict, dataclass, field
 from enum import StrEnum
 from pathlib import Path
 from typing import Generator, Iterable, NamedTuple, Self
@@ -72,11 +72,40 @@ def batched(iterable: Iterable, n: int) -> Generator[tuple, None, None]:
         yield batch
 
 
+@dataclass(frozen=True)
+class Arch:
+    # The architecture family, e.g. "x86_64", "aarch64".
+    family: str
+    # The architecture variant, e.g., "v2" in "x86_64_v2"
+    variant: str | None = None
+
+    def key(self) -> str:
+        return str(self)
+
+    def __str__(self) -> str:
+        return (self.family + "_" + self.variant) if self.variant else self.family
+
+    def __gt__(self, other) -> bool:
+        return (self.family, self.variant or "") > (other.family, other.variant or "")
+
+    def __lt__(self, other) -> bool:
+        return (self.family, self.variant or "") < (other.family, other.variant or "")
+
+
+type PlatformTripleKey = tuple[str, str, str]
+
+
 class PlatformTriple(NamedTuple):
+    # The operating system, e.g. "linux", "macos", "windows".
     platform: str
-    arch: str
+    # The architecture, e.g. "x86_64", "aarch64".
+    arch: Arch
+    # The libc implementation, e.g. "gnu", "musl", "none".
     libc: str
 
+    def key(self) -> PlatformTripleKey:
+        return (self.platform, self.arch.key(), self.libc)
+
 
 class Version(NamedTuple):
     major: int
@@ -229,12 +258,12 @@ class CPythonFinder(Finder):
         downloads = []
         for version_downloads in downloads_by_version.values():
             selected: dict[
-                tuple[PlatformTriple, Variant | None],
+                tuple[PlatformTripleKey, Variant | None],
                 tuple[PythonDownload, tuple[int, int]],
             ] = {}
             for download in version_downloads:
                 priority = self._get_priority(download)
-                existing = selected.get((download.triple, download.variant))
+                existing = selected.get((download.triple.key(), download.variant))
                 if existing:
                     existing_download, existing_priority = existing
                     # Skip if we have a flavor with higher priority already (indicated by a smaller value)
@@ -247,7 +276,7 @@ class CPythonFinder(Finder):
                             existing_download.flavor,
                         )
                         continue
-                selected[(download.triple, download.variant)] = (
+                selected[(download.triple.key(), download.variant)] = (
                     download,
                     priority,
                 )
@@ -368,11 +397,12 @@ class CPythonFinder(Finder):
 
         return PlatformTriple(operating_system, arch, libc)
 
-    def _normalize_arch(self, arch: str) -> str:
+    def _normalize_arch(self, arch: str) -> Arch:
         arch = self.ARCH_MAP.get(arch, arch)
         pieces = arch.split("_")
-        # Strip `_vN` from `x86_64`
-        return "_".join(pieces[:2])
+        family = "_".join(pieces[:2])
+        variant = pieces[2] if len(pieces) > 2 else None
+        return Arch(family, variant)
 
     def _normalize_os(self, os: str) -> str:
         return os
@@ -472,8 +502,8 @@ class PyPyFinder(Finder):
 
         return list(results.values())
 
-    def _normalize_arch(self, arch: str) -> str:
-        return self.ARCH_MAPPING.get(arch, arch)
+    def _normalize_arch(self, arch: str) -> Arch:
+        return Arch(self.ARCH_MAPPING.get(arch, arch), None)
 
     def _normalize_os(self, os: str) -> str:
         return self.PLATFORM_MAPPING.get(os, os)
@@ -539,7 +569,7 @@ def render(downloads: list[PythonDownload]) -> None:
         )
         results[key] = {
             "name": download.implementation,
-            "arch": download.triple.arch,
+            "arch": asdict(download.triple.arch),
             "os": download.triple.platform,
             "libc": download.triple.libc,
             "major": download.version.major,