The main goal of these changes is to make test_pickle.py
more readable and eliminate get_pickles.py.

These changes:
  - add dict_data, pickled_data, and pickle_file_path fixtures
  - rename test_pickle to test_unpickle
  - add test_pickle_format_stability
  - replace test_load_from_file with test_pickle_backward_compatibility
  - replace get_pickles.py with test_write_pickle_file