Bump version to 0.0.21

Update the project version in pyproject.toml from 0.0.20 to 0.0.21.

Author: baseplate-admin

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "django-hstore-widget"
-version = "0.0.20"
+version = "0.0.21"
 description = "Human friendly HStoreWidget. Continual of django-admin-hstore-widget."
 readme = "README.md"
 license = { text = "MIT" }