Use responses package for mocking the server

[chrisjsewell]

Here:

aiida-firecrest/tests/conftest.py

Line 52 in eb7f751

def firecrest_server(

you can see I directly monkeypatch requests, to mock the server, but using https://github.com/getsentry/responses is probably better

This kind of thing may also may also be good to upstream to pyfirecrest, or at least have a common solution