www/py-notebook: Update to 6.1.5, Fix security issue

Changelog: https://github.com/jupyter/notebook/blob/6.1.5/docs/source/changelog.rst

Approved by: python (with hat)
MFH: 2020Q4 (blanket, release for security issue)
Security: cf39ddf8-21be-11eb-8b47-641c67a117d8