Update supported python version from 2.7.1 to 2.7.3

The build system has been tested and developed against 2.7.3, more work
would be required to ensure the build system could be used transpartently
with any of the 2.7.x python source tree.

Co-authored-by: Pat Marion <pat.marion@kitware.com>

Fixes #21
Merge branch '21-fix-version-in-readme'

* 21-fix-version-in-readme:
  Update supported python version from 2.7.1 to 2.7.3