Add install requires

[oleksandr-pavlyk]

This PR adds install_requires=["numpy", "dpcpp_cpp_rt"] keyword argument to setup call.

It also adds requirements.txt and docs/requirements.txt per #398

Imposing minimal version requirement on dpcpp_cpp_rt does not appear possible in setup.py because install_requires must be provided before the build_backend script has a chance to run.

We should add a Python visible function to get the output of DPCTLService_GetDPCPPVersion().

[coveralls]

Coverage remained the same at 65.067% when pulling a0929ce on add-install-requires into 73440ac on master.