www/py-requests: Update to 2.17.3

• Update PORTVERSION and distinfo checksum (2.17.3)
• Add RUN_DEPENDS (recently de-vendored/de-bundled)
• Add more TEST_DEPENDS that exist in ports
• Patch out pytest-cov from test_requirements (unnecessary)
• Mark concurrent Python version safe
• Update LICENSE (ISCL -> APACHE20) [1]

https://github.com/requests/requests/blob/v2.17.3/HISTORY.rst

PR: 218578
Submitted by: <takefu airport fm> [1]