Prepare release v6.4.0 (#551)

## heroku/python

### Added

- Added support for Ubuntu 26.04 (and thus Heroku-26 / `heroku/builder:26`). ([#550](#550))

Co-authored-by: heroku-linguist[bot] <136119646+heroku-linguist[bot]@users.noreply.github.com>

Author: heroku-linguist[bot]

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [6.4.0] - 2026-04-09
+
 ### Added
 
 - Added support for Ubuntu 26.04 (and thus Heroku-26 / `heroku/builder:26`). ([#550](https://github.com/heroku/buildpacks-python/pull/550))
@@ -627,7 +629,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Initial implementation. ([#3](https://github.com/heroku/buildpacks-python/pull/3))
 
-[unreleased]: https://github.com/heroku/buildpacks-python/compare/v6.3.0...HEAD
+[unreleased]: https://github.com/heroku/buildpacks-python/compare/v6.4.0...HEAD
+[6.4.0]: https://github.com/heroku/buildpacks-python/compare/v6.3.0...v6.4.0
 [6.3.0]: https://github.com/heroku/buildpacks-python/compare/v6.2.0...v6.3.0
 [6.2.0]: https://github.com/heroku/buildpacks-python/compare/v6.1.0...v6.2.0
 [6.1.0]: https://github.com/heroku/buildpacks-python/compare/v6.0.1...v6.1.0

buildpack.toml

@@ -2,7 +2,7 @@ api = "0.10"
 
 [buildpack]
 id = "heroku/python"
-version = "6.3.0"
+version = "6.4.0"
 name = "Heroku Python"
 homepage = "https://github.com/heroku/buildpacks-python"
 description = "Heroku's buildpack for Python applications."