Add Cython/C file timestamp checking to checklist.py

~~**Not ready for merge!** Hopefully the tests will fail, as I updated a .pyx file without regenerating the C code.~~

Added git timestamp checking of Cython and generated C files to ensure that Cython files are never newer than their generated C file. This check helps prevent a developer from modifying a .pyx file without committing a regenerated C file.

Partially addresses #461.