Python Buildpack: Support pip detection of Apache Airflow required environment variable

Change effective on 09 October 2018

Apache Airflow fails to install if environment variable SLUGIFY_USES_TEXT_UNIDECODE is not set during pip install. This is problematic for the buildpack as environment variables are not accessible to the pip process by default.

The Python Buildpack now checks for this environment variable and sets it during the pip install process if present in the user’s application environment.