Bitarray

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* use `Py_SET_TYPE()` and `Py_SET_SIZE()` for Python 3.10, 109
* add official Python 3.10 support
* fix slice assignment to same object,
e.g. `a[2::] = a` or `a[::-1] = a`, 112
* add bitarray.h, 110

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* use PyType_Ready for all types: bitarray, bitarrayiterator,
decodeiterator, decodetree, searchiterator

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* add `decodetree` object, for speeding up consecutive calls
to `.decode()` and `.iterdecode()`, in particular when dealing
with large prefix codes, see 103
* add optional parameter to `.tolist()` which changes the items in the
returned list to integers (0 or 1), as opposed to Booleans
* remove deprecated `bitdiff()`, which has been deprecated since version
1.2.0, use `bitarray.util.count_xor()` instead
* drop Python 2.6 support
* update license file, 104

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* add optional index parameter to `.index()` to invert single bit
* fix `sys.getsizeof(bitarray)` by adding `.__sizeof__()`, see issue 100

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* add PyType_Ready usage, issue 66
* speedup search() for bitarrays with length 1 in sparse bitarrays,
see issue 67
* add tests

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* support signed integers in `util.ba2int()` and `util.int2ba()`,
see issue 85
* deprecate `.length()` in favor of `len()`