The `__future__` we relied on is now, where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):
* absolute_import
* print_function
* unicode_literals
* division
These import statements are no-ops and are no longer necessary.
The library `six` provides compatibility between Python 2, and 3. It's no
longer necessary once we migrate of Python 2 completely.
Also remove any custom logic for Python 2 (the ones referenced by
a commentanyways).
https://bugs.swift.org/browse/SR-16025
Previously, we were using backtrace_symbol and then parsing/modifying its
output. By just using dladdr directly, we have a cleaner and more robust
solution.
rdar://25064742
