lang/python36: Update to 3.6.15

Changelog:

https://docs.python.org/release/3.6.15/whatsnew/changelog.html#changelog

PR: 258518
Reported by: Mike Fisher
Approved by: kai (python, maintainer)
MFH: 2021Q3
Security: 0e561173-0fa9-11ec-a2fa-080027948c12