Fix import warning

[lambdamusic]

While running the tests

Query #3... returning dataframe
<dimcli.Result object #4345229152: [('year', 20), ('_stats', 1)]>
Query results:
 ==> res.json.keys():  dict_keys(['year', '_stats'])
 ==> res.stats:  {'total_count': 164463}
/Users/michele.pasin/Envs/dimcli_py3/lib/python3.7/distutils/__init__.py:1: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
/Users/michele.pasin/Envs/dimcli_py3/lib/python3.7/importlib/_bootstrap.py:219: ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__
  return f(*args, **kwds)
 ==> res.as_dataframe():      count    id

Possibly related: https://stackoverflow.com/questions/52596204/the-imp-module-is-deprecated