Parallelized Build / CI
Modeled after https://github.com/cython/cython/pull/3187 which in 0.29.14 added parallel support

A very slight boost to timing from a clean directory (bottleneck is in the Cythonize calls in both cases)

```sh
# Master
$ time python setup.py build_ext --inplace -j 4 : 
62.93s user 3.18s system 97% cpu 1:07.77 total

# this branch
$ time python setup.py build_ext --inplace -j 4:
58.87s user 3.32s system 105% cpu 59.030 total
```

