* NumPy's C extension modules have begun to use multi-phase initialisation,
  as defined by :pep:`489`. As part of this, a new explicit check has been added
  that each such module is only imported once per Python process. This comes with
  the side-effect that deleting ``numpy`` from ``sys.modules`` and re-importing
  it will now fail with an ``ImportError``. This has always been unsafe, with
  unexpected side-effects, though did not previously raise an error.