setup.py maintenance

[pavlin-policar]

Issue

It seems that in the latest version of pip, we can no longer pass in a class into include_dirs that eventually resolves into a string. This was previously done because running setup.py in a fresh environment would fail as numpy had not yet been installed. Therefore, we used a utility get_numpy_include class which would import numpy only when it was called, after numpy was already installed. This is apparently no longer supported.

Related issue: #248
Failing tests in PR #246

Description of changes

This is most likely no longer needed because we now build using pyproject.toml, where we can pre-specify the build requirements, which will always be installed before the setup.py script is run. Therefore, we no longer need any checks if numpy is installed.
Similarly, the have_cython check is no longer needed in setup.py either, as we specify cython as a build dependency in pyproject.toml.

Includes

• Code changes
• Tests
• Documentation