MFH: r445845 www/py-requests: Update to 2.18.1

While I'm here, add COMMENT on specific supported Python versions

[1] https://github.com/requests/requests/pull/4192

QA: 3 failed [1], 480 passed, 11 skipped, 2 xpassed in 84.60 seconds

Approved by: ports-secteam (blanket: bugfix)