Close smarts file handle after reading

[iprotsyuk]

This PR fixes the following warning:

dimorphite_dl/dimorphite_dl.py:718: ResourceWarning: unclosed file <_io.TextIOWrapper name='dimorphite_dl/site_substructures.smarts' mode='r' encoding='UTF-8'>
  for l in open(site_structures_file, "r")
ResourceWarning: Enable tracemalloc to get the object allocation traceback