Declare supported Python version

[Freso]

Right now we just state that "Whipper is a Python 2 CD-DA ripper", but doesn't state which Python 2(s).

We should probably declare somewhere what version(s) of Python we (intend to) support. I would suggest either:

• 2.7(.x): to ease the upgrade path to Python 3, and because Python 2 is going to become unsupported in a couple of years entirely anyway, so Py<2.7 is a deadend), or
• 2.7.9+: Python 2.7.9 is the earliest Python 2 version to support SNI, which is used by musicbrainz.org. As we're not using requests or another 3rd party library to handle HTTP stuff, using Python <2.7.9's standard handling of this might lead to unexpected errors on platforms we claim to support.

The first "looks prettier" but also suggests that 2.7.0 should work. 2.7.9+ is a bit more verbose, but denotes that whipper may not work well with older Pythons. However, Ubuntu Trusty (14.04 LTS) ships with Python 2.7.6, so there will still be a number of wild Python <2.7.9's out there for a while to come.

All this said, Python 2 will be unsupported from Python itself in 2020, 3 years from now, so maybe this doesn't matter much and we should just focus on moving to Python 3 ASAP (#78).