Test dill
_The documentation is not available anymore as the PR was closed or merged._ I should point out that the hash will be the same if computed twice with the same code on the same version of dill (after adding huggingface's code that removes line numbers and file names, and sorts globals.) My changes in dill 0.3.5 and ones that I will make in 0.3.6 will result in different pickles than the ones dill 0.3.4 was making. This should still be fine for caching. Just some comments @lhoestq:

The best practice for testing is to have a `test_<filename>.py` for each `<filename>.py`. Therefore in order to have the filenames aligned, I would propose:
- either renaming `fingerprint.py` to `caching.py`
- or renaming `test_caching.py` to `test_fingerprint.py`

On the other hand, my idea when implementing this test was not to test all the functionalities of the `Hasher`, but just to have a regression test that fails if dill version is > 0.3.4 and the pin in our `setup.py` is not present. Just recall that we had no failing test in our CI when the issue with dill was found on `transformers`.

The objective of this PR is just to have a regression test for that case: I tested and I got `AttributeError: module 'dill._dill' has no attribute 'stack'`

For this regression test, I took into account this comment by @gugarosa: https://github.com/huggingface/datasets/issues/4379#issuecomment-1133131825

There is no equivalent test in `test_caching.py` because our CI did not fail before pinning dill. Ok I see, renaming it to `test_fingerprint.py` sounds like a good idea :)
Regression test for future releases of `dill`.

Related to #4379. 