Update python requirements to 2.7 or >=3.5
We should update the python version requirements to allow for 2.7 or >=3.5
. Currently the checks are all a simple >=2.6
, and are individually stated for each subpackage, with extra requirements for Doxygen and Swig, so we should probably hardcode the requirements somewhere, or separate out python2 and python3 version checks.