[python-committers] Making the PSF CoC apply to core developers

I noticed that the devguide didn't explicitly mention that core developers were expected to follow the PSF CoC ( https://docs.python.org/devguide/coredev.html and https://www.python.org/psf/codeofconduct/, respectively). I have opened http://bugs.python.org/issue26446 to make sure it gets documented.

Since this is technically a modification of the requirements of getting commit privileges I wanted to mention it here before I (or anyone else) made the change.