PyO3 backend

[termoshtt]

Resolve #23

• It is hard to support both PyO3 and rust-cpython interface.
  • rust-numpy 0.2.* uses rust-cpython, and will be deprecated after PyO3 can be compiled with stable Rust Compile with stable rust pyo3#5
  • rust-numpy 0.3 uses PyO3

TODO

• Replace rust-cpython -> PyO3
• Adopt to PyO3 interface
• Fix CI (since PyO3 needs nightly)

[termoshtt]

error[E0277]: the trait bound `numpy::PyArray: pyo3::FromPyObject<'_>` is not satisfied
error[E0277]: the trait bound `numpy::PyArray: pyo3::IntoPyObject` is not satisfied

These should be addressed.

[termoshtt]

Use pyobject_* macros exposed in PyO3/pyo3#155, and revised in my branch

The exposed macro works fine. But PyArray_Type and PyArray_Check are still incompatible with PyO3.

[dthul]

It would be awesome if this worked. Anything you could use help with?