Fix numpy dependency

[mt-krainski]

Taking over #158.

This fixes the installation process so that installing nnef-tools will now also install numpy as it is a build and runtime dependency. This can now be installed in virtual environments without manually specifying the dependency on numpy as well as with other package managers e.g. poetry.

I locked the numpy version to be above 1.13 because that's when __NUMPY_SETUP__ became deprecated. If this was to support older versions of numpy, we'd have to add a try-except block to check for __NUMPY_SETUP__ in finalize_options. We can also lock the version higher.

setup_requires is required, without it the build doesn't work - I think with setup_required, numpy is installed before building nnef-tools. Without it it might be installed after.

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[gyenesvi]

@mt-krainski, so this is to replace #158 right? I like this solution, it looks cleaner to me. So if we merge this one, we can drop the other one, right?

[mt-krainski]

That is correct!