Use Python 3 build dependencies

https://docs.platform.sh/configuration/app/build.html#python-dependencies

.platform.app.yaml

@@ -11,7 +11,7 @@ type: 'python:3.7'
 
 # The build-time dependencies of the app.
 dependencies:
-    python:
+    python3:
        pipenv: '*'
 
 # The relationships of the application with services or other applications.