Update Poetry section

[smangham]

• Poetry now uses a different script location - the existing instructions give a warning.
• We pin numpy to version 1.20, but this conflicts with some versions of Python (possibly specifically the version we pin? Need to double-check this). One learner suggested we make this a learning point and tell people to solve the problem (i.e. change the pinned number) but I think for clarity just changing it makes more sense.