Orderedset

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* bugfix: Generate new C file to fix compile issues

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* bugfix: Fix deprecation warning for collections.abc in Python 3.8+

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* bugfix: Fix `isdisjoint` to return True when the sets are disjoint
* build: Include 3.6 when testing
* dist: Include test files in sdist
* docs: Make the Readme a bit prettier

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* breaking change: All comparisons, other than `eq`, against other ordered sets
are now performed unordered; i.e., they are treated as regular sets.
* `isorderedsubset` and `isorderedsuperset` have been added to perform ordered
comparisons against other sequences. Using these methods with unordered
collections wield yield arbitrary (and depending on Python implementation,
unstable) results.

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* bugfix: Set operations only worked with iterables if the OrderedSet was on the
left-hand side. They now work both ways.
* bugfix: The order of an intersection was the right-hand side's order. It is now
fixed to be the left-hand side's order.

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* Make comparisons work with sets and lists, and not crash when compared with None.