avoid loading hoc.so in mac that raises symbol not found #417

[anilbey]

Fixes #417 that raises the following error on macos.

  File "env_path/lib/python3.9/ctypes/__init__.py", line 382, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: dlopen(env_path/lib/python3.9/site-packages/neuron/hoc.cpython-39-darwin.so, 0x0006): symbol not found in flat namespace '_modl_reg'