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)