Strategy to publish external modules for different versions of ITK #142

Currently, it is tricky to know which version of an ITK external module can be used with a specific version of ITK.
Since often external modules are not updated on PyPi, users need to try until they find a version of ITK that (seems to) work.

It would be nice if there was a consistent strategy. Some ideas:

• in setup.py only the current minor versions are allowed, e.g. itk >= 5.3.0, < 5.4.0
• in setup.py only the current patch versions are allowed, e.g. itk >= 5.2.1, < 5.2.2