Refactor fetch-download-metadata.py script (#4853)

## Summary

Similiar to https://github.com/astral-sh/rye/pull/680, I have made a
major refactor to the `fetch-download-metadata.py` script.

Some notable changes:

- Use PEP 723 inline scripts
- Fully type annotated the script
- Implemented async HTTP fetching
- Introduced a `Finder` base class and move finder logic under
`CPythonFinder` subclass, which will make it easier to add a
`PyPyFinder` later.
- Instead of fetching `xxx.sha256` for each file, the script now fetches
a single `SHA256SUMS` file containing checksums for all files in the
release.

As a result, the script now takes around 10 seconds instead of 10+
minutes.

## Plan for Future PRs

- [ ] Implement the `PyPyFinder`
- [ ] Add an GitHub Action to run `fetch-download-metadata.py` daily and
create PR automatically

## Test Plan

```sh
cargo run -- run --isolated -- ./crates/uv-python/fetch-download-metadata.py
```

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

@@ -1,4 +1,9 @@
-#!/usr/bin/env python3.12
+# /// script
+# requires-python = ">=3.12"
+# dependencies = [
+#   "chevron-blue < 1",
+# ]
+# ///
 """
 Generate static Rust code from Python version download metadata.
 
@@ -6,7 +11,7 @@ Generates the `downloads.inc` file from the `downloads.inc.mustache` template.
 
 Usage:
 
-    uv run --isolated --with chevron-blue -- crates/uv-python/template-download-metadata.py
+    uv run --isolated -- crates/uv-python/template-download-metadata.py
 """
 
 import sys
@@ -16,6 +21,8 @@ import json
 import subprocess
 from pathlib import Path
 
+import chevron_blue
+
 CRATE_ROOT = Path(__file__).parent
 WORKSPACE_ROOT = CRATE_ROOT.parent.parent
 VERSION_METADATA = CRATE_ROOT / "download-metadata.json"
@@ -23,16 +30,6 @@ TEMPLATE = CRATE_ROOT / "src" / "downloads.inc.mustache"
 TARGET = TEMPLATE.with_suffix("")
 
 
-try:
-    import chevron_blue
-except ImportError:
-    print(
-        "missing requirement `chevron-blue`",
-        file=sys.stderr,
-    )
-    exit(1)
-
-
 def prepare_name(name: str) -> str:
     match name:
         case "cpython":
@@ -73,8 +70,8 @@ def main():
     debug = logging.getLogger().getEffectiveLevel() <= logging.DEBUG
 
     data = {}
-    data["generated_with"] = Path(__file__).relative_to(WORKSPACE_ROOT)
-    data["generated_from"] = TEMPLATE.relative_to(WORKSPACE_ROOT)
+    data["generated_with"] = Path(__file__).relative_to(WORKSPACE_ROOT).as_posix()
+    data["generated_from"] = TEMPLATE.relative_to(WORKSPACE_ROOT).as_posix()
     data["versions"] = [
         {"key": key, "value": prepare_value(value)}
         for key, value in json.loads(VERSION_METADATA.read_text()).items()