Bump the minimum Python version requirement to 2.7
    
    Older Python versions are long since past their EOL, we don't need to
    support them either. Python 2.7 is also the least incompatible version
    compared to Python 3, going forward. Nuke the now unnecessary compat
    macros.
    
    (cherry picked from commit 3f3cb3eabf7bb49dcc6e691601f89500b3487e06)