Provide base for modernisation

[clbarnes]

• add dependency version constraints to requirements.txt per Not compatible with numpy>=1.12 #6 Not compatible with scipy >=1.3.0 #8
• allow test script to run on non-EOL python versions
• add CI for non-EOL python versions (they all fail)

Suggested: add a pyproject.toml to allow package to be built by pip without pre-installing compile-time dependencies (see PEP517/518). That would look like

[build-system]
requires = ["setuptools", "cython>=0.24", "numpy>=1.9,>1.12", "scipy<1.3", "wheel"]
build-backend = "setuptools.build_meta"

but given there are 2 layers of setup.py, I'm not sure which directory it would go into.

I haven't worked with C/C++/cython extensions before so I don't know how to fix the failures, but this at least provides a way to consistently test them.