@rflamary Given these changes, we might be able to roll-back the numpy requirement bump made in #326. If POT doesn't use anything special (at build time) from numpy==1.20.0, we could allow oldest-numpy-version to compile against even older versions (by decreasing the minimum Python version requirement in python_requires of setup.py, which, in turn, would allow older numpy versions).

The reason I introduced the minimum Python version / python_requires>=3.7 in the first place, is to respect the already merged precedent that Python 3.6 support is dropped.

But it looks like it might have been dropped pre-maturely due to the nature of the numpy>=1.20 build-time constraint that is now improved in this PR. I realize that a Python 3.6 runtime can still use POT by compiling it from source, but the proper way to specify what versions of Python you support is by using python_requires. I'd be happy to bump python_requires down to >=3.6 and re-enable wheel building for cp36, now that wheels will be properly built against the proper numpy version. Let me know what you think.