pip install .[io] not working due to hyperspy depending on traits (no wheels on PyPI)

Currently, pip install .[io] is not working, due to hyperspy depending on traits, which does not have precompiled wheels on PyPI. The test_install job does not work without a wheel, or without getting Microsoft Visual C++ 14 for its gcc (which is a large download and would complicate gitlab-ci.yml).

The issue is known to the HyperSpy guys and traits also has a request to build wheels for PyPI, but it does not look like this will be fixed in the near future...

For now, hyperspy is removed from [all], if people want to use hyperspy for io, they need to install it with conda for now (or get a wheel for Windows from gohlke.

bug

UPDATE: mayavi has the same problem and tvtk is also not easily installable via pip. As it stands, the [all] is quite empty...