Add mypy type check for uv-python scripts (#5332)

## Summary

Per https://github.com/astral-sh/uv/pull/4853#issuecomment-2212505407

> If we're going to aim for full type coverage, we should probably
follow this by adding type checking in CI too otherwise it seems too
easy for it to become out of date.

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

@@ -78,7 +78,7 @@ class ImplementationName(StrEnum):
 class PythonDownload:
     version: Version
     triple: PlatformTriple
-    flavor: str | None
+    flavor: str
     implementation: ImplementationName
     filename: str
     url: str
@@ -185,7 +185,7 @@ class CPythonFinder(Finder):
         # Collapse CPython variants to a single URL flavor per triple
         downloads = []
         for choices in results.values():
-            flavors = {}
+            flavors: dict[PlatformTriple, tuple[PythonDownload, int]] = {}
             for choice in choices:
                 priority = self._get_flavor_priority(choice.flavor)
                 existing = flavors.get(choice.triple)
@@ -250,12 +250,12 @@ class CPythonFinder(Finder):
         # Ex)
         # https://github.com/indygreg/python-build-standalone/releases/download/20240107/cpython-3.12.1%2B20240107-aarch64-unknown-linux-gnu-lto-full.tar.zst
         if url.endswith(".sha256"):
-            return
+            return None
         filename = unquote(url.rsplit("/", maxsplit=1)[-1])
 
         match = self._filename_re.match(filename)
         if match is None:
-            return
+            return None
 
         version, triple = match.groups()
         if triple.endswith("-full"):
@@ -265,14 +265,14 @@ class CPythonFinder(Finder):
         if match is not None:
             triple, flavor = match.groups()
         else:
-            flavor = None
+            flavor = ""
         if flavor in self.HIDDEN_FLAVORS:
-            return
+            return None
 
         version = Version.from_str(version)
         triple = self._normalize_triple(triple)
         if triple is None:
-            return
+            return None
 
         return PythonDownload(
             version=version,
@@ -286,7 +286,7 @@ class CPythonFinder(Finder):
     def _normalize_triple(self, triple: str) -> PlatformTriple | None:
         if "-static" in triple:
             logging.debug("Skipping %r: static unsupported", triple)
-            return
+            return None
 
         triple = self.SPECIAL_TRIPLES.get(triple, triple)
         pieces = triple.split("-")
@@ -300,7 +300,7 @@ class CPythonFinder(Finder):
                 libc = "none"
         except IndexError:
             logging.debug("Skipping %r: unknown triple", triple)
-            return
+            return None
 
         return PlatformTriple(arch, operating_system, libc)
 
@@ -313,7 +313,7 @@ class CPythonFinder(Finder):
     def _normalize_os(self, os: str) -> str:
         return os
 
-    def _get_flavor_priority(self, flavor: str | None) -> int:
+    def _get_flavor_priority(self, flavor: str) -> int:
         """Returns the priority of a flavor. Lower is better."""
         try:
             pref = self.FLAVOR_PREFERENCES.index(flavor)
@@ -385,7 +385,7 @@ async def find() -> None:
     render(downloads)
 
 
-def main():
+def main() -> None:
     parser = argparse.ArgumentParser(description="Fetch Python version metadata.")
     parser.add_argument(
         "-v",

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

@@ -14,12 +14,13 @@ Usage:
     uv run --isolated -- crates/uv-python/template-download-metadata.py
 """
 
-import sys
-import logging
 import argparse
 import json
+import logging
 import subprocess
+import sys
 from pathlib import Path
+from typing import Any
 
 import chevron_blue
 
@@ -66,10 +67,10 @@ def prepare_value(value: dict) -> dict:
     return value
 
 
-def main():
+def main() -> None:
     debug = logging.getLogger().getEffectiveLevel() <= logging.DEBUG
 
-    data = {}
+    data: dict[str, Any] = {}
     data["generated_with"] = Path(__file__).relative_to(WORKSPACE_ROOT).as_posix()
     data["generated_from"] = TEMPLATE.relative_to(WORKSPACE_ROOT).as_posix()
     data["versions"] = [