Python 3 selection

[Hasimir]

• At this stage it is more than reasonable to select Python 3 and no longer support Python 2.7 (especially with that usage exit message), not to mention 2.7 hitting EOL in 2020 (just like Roy Batty, it will be gone ... like tears in rain ...).
• Included a check to stick with 3.3 or above. It'd probably work with 3.2, but the major changes, IIRC, were relating to Unicode handling and given some of the bad encoding in some of the PGN files floating around anything that prevents trying to troubleshoot character encoding clashes is a Good Thing™.
• I'm pretty sure it should be mathematically impossible to hit the final "else" part of that check.
• The script is too simple to give it argparse flags, but that can always be misappropriated from one of my dodgier projects (dodgy by content, not code quality).

[niklasf]

Thanks.

I cherry-picked the first line. The rest is really not nescessary. Python 2 won't run the script anyway. Python 3.2 should work fine - new in Python 3.3 is just a notation for literals it brings back from Python 2.

[Hasimir]

fair enough, I get a bit silly when awake too long and at least 5 lines were the result of that. ;)