Switch to AWS' dual-stack S3 URLs (#513)

The default AWS S3 URLs only support IPv4, whereas the dual-stack
URLs support both IPv4 and IPv6:
https://docs.aws.amazon.com/AmazonS3/latest/API/ipv6-access.html
https://docs.aws.amazon.com/AmazonS3/latest/API/dual-stack-endpoints.html

By switching to the dual-stack URLs, it means systems that support IPv6
will now use it instead of IPv4, which is helpful in IPv6 environments where
IPv4 traffic has to fall back to being routed via NAT gateways (which has
performance and ingress cost implications).

See also:
https://salesforce-internal.slack.com/archives/C01R6FJ738U/p1770827176094169

GUS-W-21336806.

Author: edmorley

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Changed the S3 URL used to download Python to use AWS' dual-stack (IPv6 compatible) endpoint. ([#513](https://github.com/heroku/buildpacks-python/pull/513))
+
 ## [5.0.0] - 2026-02-10
 
 ### Changed

src/python_version.rs

@@ -99,7 +99,7 @@ impl PythonVersion {
             ..
         } = target;
         format!(
-            "https://heroku-buildpack-python.s3.us-east-1.amazonaws.com/python-{major}.{minor}.{patch}-{distro_name}-{distro_version}-{arch}.tar.zst"
+            "https://heroku-buildpack-python.s3.dualstack.us-east-1.amazonaws.com/python-{major}.{minor}.{patch}-{distro_name}-{distro_version}-{arch}.tar.zst"
         )
     }
 }
@@ -231,7 +231,7 @@ mod tests {
                 distro_name: "ubuntu".to_string(),
                 distro_version: "22.04".to_string()
             }),
-            "https://heroku-buildpack-python.s3.us-east-1.amazonaws.com/python-3.11.0-ubuntu-22.04-amd64.tar.zst"
+            "https://heroku-buildpack-python.s3.dualstack.us-east-1.amazonaws.com/python-3.11.0-ubuntu-22.04-amd64.tar.zst"
         );
         assert_eq!(
             PythonVersion::new(3, 12, 2).url(&Target {
@@ -241,7 +241,7 @@ mod tests {
                 distro_name: "ubuntu".to_string(),
                 distro_version: "24.04".to_string()
             }),
-            "https://heroku-buildpack-python.s3.us-east-1.amazonaws.com/python-3.12.2-ubuntu-24.04-arm64.tar.zst"
+            "https://heroku-buildpack-python.s3.dualstack.us-east-1.amazonaws.com/python-3.12.2-ubuntu-24.04-arm64.tar.zst"
         );
     }