devel/py-futures: Update to 3.0.5

• Update PORTVERSION and distinfo checksum (3.0.5)
• Correctly limit to Python versions < 3. :2 means depend on lang/python2 (for the python2 symlink), which is an incorrect specification.
• Enable NO_ARCH (architecture independent)
• Add a comment above the test target to inform our future selves to increase the number of POSIX semaphores using sysctl. FreeBSD currently defaults to 30, and I don't believe there's a "blessed" way to have tests run as root to do this programatically.
  
  Without increasing the value, many tests are skipped:
  
  skipped 'system provides too few semaphores (30 available, 256 necessary)'

Changes:

https://github.com/agronholm/pythonfutures/blob/3.0.5/CHANGES