addition of pyproject.toml broke mpi parallel builds

[jswhit]

PR #1208 added pyproject.toml and removed the deprecated build_requires. However, since mpi4py.get_include is used when building with MPI parallel support, and mpi4py cannot be included in pyproject.toml (since it has no wheels on pypi), this broke the mpi tests. As a workaround, PR #1210 adds --no-build-isolation to the pip install so that mpi4py can be imported by setup.py.

[ocefpaf]

Ouch! Sorry for not catching that. I'm not sure if there is a better solution, it seems that h5py does the same.