Updated build process for new pip install

[williamjameshandley]

Description

Trying to fix broken build associated with new polychord install process with pip. Should also update the wiki when this is fixed

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 pybambi tests)
• My code contains compliant docstrings (sphinx-build docs/source docs/build -nW; pydocstyle pybambi)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works