pot installation fails with numpy

pot installation fails with numpy < 1.20

Describe the bug

import ot fails with after pip install pot with numpy<=1.19.5


---> 22 from .emd_wrap import emd_c, check_result, emd_1d_sorted
     23 from .solver_1d import emd_1d, emd2_1d, wasserstein_1d
     24 

ot/lp/emd_wrap.pyx in init ot.lp.emd_wrap()

ValueError: numpy.ndarray size changed, may indicate binary incompatibility. Expected 88 from C header, got 80 from PyObject

Fixes

Either we re-do the wheels with earlier versions of numpy or we require numpy >= 1.20. Since 1.20 however, numpy has dropped support of Python 3.6.