I am not a Mac user and therefore do not use Homebrew, so take the below with that in mind. Have you tried?:
brew install petere/postgresql/postgresql@11 --with-python
Great idea!! It works.
This didn’t work:
brew install petere/postgresql/postgresql@11 --with-python
Probably because options are no longer permitted - but this did:
brew install petere/postgresql/postgresql@11
Since the —with-python option was part of the script, the plpythonu extension is available in the server.