chore(ci): explicitly use `pypy-3.7` for PyPy
chore(ci): explicitly use `pypy-3.7` for PyPy

This is required since v3 of `actions/setup-python` (https://github.com/actions/setup-python/releases/tag/v3.0.0).

python-poetry/poetry-core,python-poetry/poetry-core,python-poetry/poetry-core