PY34 is defined inconsistently compared to PY2 and PY3

[benjaminp]

Originally reported by: astrofrog (Bitbucket: astrofrog, GitHub: astrofrog)

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The constant PY34 is badly named: PY2 means Python 2, PY3 means Python 3, but PY34 will be True for Python 3.6 or Python 4.3, which will read weirdly in code.

I would therefore propose renaming PY34 to PY_GE_34, and introducing a new variable PY_GE_3 to mean sys.version_info[0] >= 3.

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• Bitbucket: https://bitbucket.org/gutworth/six/issue/147