better specification of supperted python versions [ci skip]
make tests compatible with python 3
add python3 badge [ci skip]
fix future imports
2to3 likes a space between the future imports and the rest... [ci skip]
rm doctest in DoubleExp, some 2to3 import changes
python 3 support for potential module
Revert "2to3 likes a space between the future imports and the rest... [ci skip]"

This reverts commit 44199c1208b368d4256f84b92b29d31bbd73149c.
dictionary compatibility with python3 for bovy_conversion
python3 compatibility for bovy_plot plus improvements
python3 dictionary compatibility for FullOrbit plus improvements
python3 dictionary compatibility for OrbitTop plus improvements
python3 dictionary compatibility for RZOrbit plus improvements
python3 dictionary compatibility for planarOrbit plus improvements
switch to explicit integer division in leapfrog
python3 dictionary compatibility for linearOrbit plus improvements
python3 dictionary compatibility for actionAngleAdiabatic plus improvements
python3 dictionary compatibility for actionAngleAxi plus improvements
python3 dictionary compatibility for actionAngleVertical
sort with key rather than with cmp