Fix CI crash on windows

[nennigb]

The CI on windows crashes since few months because the import of compiled extensions was not done properly on windows plateform. The extension is compiled using the m2w64-toolchain conda package. After the package installation a .libs is created by the building tool chain (m2w64-gfortran + msvc) and should be added to the os path.

See also

• https://pav.iki.fi/blog/2017-10-08/pywingfortran.html#building-python-wheels-with-fortran-for-windows
• https://docs.python.org/3/whatsnew/3.8.html#bpo-36085-whatsnew for python version > 3.8.