www/py-requests: Update to 2.21.0

• Update USES comment (Python 3.3 support dropped)
• Rebase setup.py patch (idna change released)
• Remove comment about failing tests due to httpbin issue which seems to now be fixed.

This update includes a pinned urllib3 version bump to < 1.25, which paves
the way for a net/urllib3 update to 1.24 [1].

Note: 2.20.0 includes a security vulnerability fix for CVE-2018-18074

Changelog:

https://github.com/requests/requests/blob/v2.21.0/HISTORY.md

PR: 229322 [1]
Security: 50ad9a9a-1e28-11e9-98d7-0050562a4d7b
MFH: 2019Q1