Setup requires numpy version incompatible with Python 3.6

Building the wheel for this package is broken for Python 3.6 because the `setup_requires` specifies numpy as `'numpy >= 1.7.0' `. This pulls in numpy `1.21.x` which is not supported with Python 3.6. Seems numpy is not resolving this constraint at install time and instead throws a runtime error.

Is it possible to specify a maximum numpy version of `1.20.x `as a setup requirements for Python 3.6?

