code: def import_best(c_module, py_module):
      
      from importlib import import_module
      from os import getenv
      pure_python = getenv("PURE_PYTHON", "")
      if pure_python:
          return import_module(py_module)
      else:
          try:
              return import_module(c_module)
          except ImportError:
              return import_module(py_module)
docs: Import the best available module,
      with C preferred to pure Python.