[ffmpeg] Move version detection to utils
[YoutubeDL] Include rtmpdump in exe versions -v output
[util] Move compatibility functions out of util

utils is large enough without these compatibility functions.

Everything that is present in newer versions of Python (i.e. with dev Python it's just an import) goes into compat.py .
Everything else (i.e. youtube-dl-specific helpers) goes into utils.py .
[compat] Fix imports
[compat] Modernize
[utils] Modernize
[test_swfinterp] Correct compilation on modern mxmlc versions