Update repo2docker/buildpacks/base.py

Co-Authored-By: nuest <daniel.nuest@wwu.de>

repo2docker/buildpacks/base.py

@@ -554,7 +554,7 @@ class BaseImage(BuildPack):
             ))
         except FileNotFoundError:
             pass
-        if {'py'}.intersection(self.stencila_contexts):
+        if 'py' in self.stencila_contexts:
             assemble_scripts.extend(
                 [
                     (