simplify skiplist inclusion/cimport to be more cythonize-friendly
Next up in the moving-towards-`cythonize` parade...

`_libs.window` has both `include "skiplist.pyx"` and `from skiplist cimport`.  The cimport refers to `src/skiplist.pxd`, which is just passing through declarations from `src/skiplist.h`.  This PR removes skiplist.pxd and moves its contents into src/skiplist.pyx.

Background on motivation:  when `cythonize` is used in setup.py it chokes on cimports from the src/ directory.  After some troubleshooting to avoid this choking, I decided to take the alternate route of just avoiding cimporting from there.  This is the first of four cimports to remove.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
