Prepare release v0.21.0 (#310)

## heroku/python

### Changed

- The build now fails early if known problematic Python and pip-related env vars have been set by the user or earlier buildpacks. ([#308](#308))
- The `PIP_PYTHON` env var is now only set at build time. ([#307](#307))

### Removed

- Stopped setting the `LANG` env var. ([#306](#306))
- Stopped setting the `PYTHONHOME` env var. ([#309](#309))

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

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.21.0] - 2024-12-18
+
 ### Changed
 
 - The build now fails early if known problematic Python and pip-related env vars have been set by the user or earlier buildpacks. ([#308](https://github.com/heroku/buildpacks-python/pull/308))
@@ -264,7 +266,8 @@ 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/v0.20.1...HEAD
+[unreleased]: https://github.com/heroku/buildpacks-python/compare/v0.21.0...HEAD
+[0.21.0]: https://github.com/heroku/buildpacks-python/compare/v0.20.1...v0.21.0
 [0.20.1]: https://github.com/heroku/buildpacks-python/compare/v0.20.0...v0.20.1
 [0.20.0]: https://github.com/heroku/buildpacks-python/compare/v0.19.2...v0.20.0
 [0.19.2]: https://github.com/heroku/buildpacks-python/compare/v0.19.1...v0.19.2

buildpack.toml

@@ -2,7 +2,7 @@ api = "0.10"
 
 [buildpack]
 id = "heroku/python"
-version = "0.20.1"
+version = "0.21.0"
 name = "Heroku Python"
 homepage = "https://github.com/heroku/buildpacks-python"
 description = "Heroku's buildpack for Python applications."