PYTHONSAFEPATH compatible setup.py

[siddharthab]

Test Plan for Python 3.11+

Without this change:

% PYTHONSAFEPATH=true python3 setup.py bdist_wheel --python-tag=py311
Traceback (most recent call last):
  File "/Users/sidb/Workspaces/python-future/setup.py", line 81, in <module>
    import src.future
ModuleNotFoundError: No module named 'src'

With this change:

% PYTHONSAFEPATH=true python3 setup.py bdist_wheel --python-tag=py311
running bdist_wheel
...