Orderly-set

- Major refactor
- Added a StableSet implementation, as a base class for OrderedSet.
- Added Many functions to OrderedSet, to be more complete and more compatible with other implementations.
- popitem(last: bool = True), similar to `dict.popitem` (note minor incompatibility with another implementation (`orderedset`) that have the `last` keyword in the `pop` function)
- move_to_end(key), similar to `dict.move_to_end`
- __le__, __lt__, __ge__, __gt__ - to improve subset/superset testing
- Minimum Python version is 3.8 (because __reversed__)
- Fix: OrderedSet.update now raised a TypeError instead of a ValueError when the type of the input is incorrect
- Added many new tests, and all the tests from 2 other implementations.

- Packaged using flit. Wheels now exist, and setuptools is no longer required.
- This package now has a typical package structure, instead of being a single module. The code is in `orderly_set/__init__.py` instead of `orderly_set.py`.
- There is an `orderly_set/py.typed` so that type checkers know about the types.
- Use the type aliases `SetLike[T]` and `OrderedSetInitializer[T]` to simplify some types.
- Updated the way overloaded type signatures are written to what MyPy currently expects.
- Minimum Python version is 3.7.

- Added type signatures inline to the code, instead of using type stubs.
- Dropped Python 2 support. The minimum supported Python version is 3.5.

- `__getitem__` accepts NumPy arrays of indices, and returns a list of elements with those indices.
- Updated in-place operations that took O(N^2) time, such as .difference_update(), to take O(N) time.
- Clarified whether various methods mutate or copy the OrderedSet.
- Added `OrderedSet.get_loc` and `OrderedSet.get_indexer` as aliases for `OrderedSet.index`, for interoperability with `pandas.Index`.
- Added type stubs in a .pyi file.

- Implemented the abstract base classes `collections.MutableSet` and `collections.Sequence`.
- Changed the behavior of some methods to follow the MutableSet API.
- Indexing an OrderedSet with `[:]` returns a copy, not the same object.

- Tuples are allowable values in the set, and are not treated as "fancy indexing".
- Added `update` and `pop` methods.