The reason was given as the label invalid, as it was an extremely minor change which was not needed.

Ok, fine for me, but take into account:

1. replacing list comprehension by generator expression is not always an "extremely" minor change, see https://www.python.org/dev/peps/pep-0289/. I could have reported a bug that the algorithm does not work on low-memory machines when n is large (Out of memory exception).
2. If this was not clear I would at least expected a request/comment to clarify instead of closing the pull request without further notice
3. where can we find an explanation of the label #invalid?

I would propose the following process improvement:
Create a guideline for administrators

• every update should be encouraged, how little it the improvement might be. Stimulating learning is the main goal of this repo.
• before merging or rejecting a pull request 2 admins have to approve (especially for admins who are new to the project)
• every rejection should be accompanied by a comment explaining why the pull request was rejected
• in case of doubt write a comment for the author and ask for further clarification