www/py-bleach: Update to 3.1.5, Fix security issue

Changelog: https://github.com/mozilla/bleach/blob/v3.1.5/CHANGES

PR: 245943
Approved by: koobs (maintainer)
MFH: 2020Q2 (security, bugfix release(s))
Security: 4c52ec3c-86f3-11ea-b5b4-641c67a117d8