Itertools: 4 updates, 13 additions and many tests

[rkompass]

updated: accumulate, chain, islice, tee; added: chain.from_iterable, combinations, combinations_with_replacement,compress, dropwhile, filterfalse, groupby, pairwise, permutations, product,takewhile, zip_longest; also added tests for all functions

[andrewleech]

Hi, I saw your comment on the other thread about the code formatting; you typically just need to run the tool that fixes it all for you and commit that.

You can run pip install -U black; python tools/codeformat.py ./path/to/my/module before committing changes to ensure the formatting matches the standard.