Teach cmake about py2.7 and py3 (py2.7 is the default)

[ermo]

This might come in handy when it comes to adding full
py3 compatibility at a later stage per issue #88 .

[Taose]

Well commented, and changes are quite simple in nature.

Test has been conducted to confirm the relevant change, code compiles and runs, change is appropriate. No further testing required.

[Taose]

Doesn't appear to cause problems that initially rendered Python3 unusable and updates to modern standards.