Add Nix code

This worked before the whole Python 3 change, but now has some minor issues I need to fix. When building hurricane, I test that it works inside Nix's sandbox by doing import Cfg to test that it's importable. This gives me the following error message:

Check whether the following modules can be imported: Viewer Hurricane Cfg Analog
PyInit_Cfg() (Python3+template)
Cfg.so loaded 0x7ffff5246c10 (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
[ERROR]
  Failed to initialize <PyLayoutGenerator>.
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<string>", line 1, in <lambda>
  File "/nix/store/97w52ckcjnfiz89h3lh7zf1kysgfm2s8-python3-3.9.6/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 666, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 565, in module_from_spec
  File "<frozen importlib._bootstrap_external>", line 1173, in create_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
SystemError: setVerboseLevel() method: bad call flags

Do you have any idea why?