Only run 3to2 during actual install steps

While installing, tools like pip first call `setup.py egg_info`,
followed by `setup.py install`. With the previous behaviour, this would
cause the sources to be run through 3to2 twice.

As a result of this, some parts of the code became mangled up, see for
example issues #232.

Another benefit of this change is that install time is reduced somewhat,
since a typical install will now only run 3to2 once instead of twice.

Fixes #232