www/py-requests: Update to 2.8.1

• Update PORTVERSION and distinfo checksum (2.8.1)
• Add NO_ARCH
• Rename test target since the framework supports TEST_DEPENDS et al.
• Add comment about failing test that's not supposed to fail (xfail)

Changes:

https://github.com/kennethreitz/requests/blob/v2.8.1/HISTORY.rst