Bitarray

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* add frozenbitarray object
* add optional start and stop arguments to `.count()` method
* add official Python 3.8 support
* optimize `setrange()` (C-function) by using `memset()`
* fix issue 74, bitarray is hashable on Python 2
* fix issue 68, `unittest.TestCase.assert_` deprecated
* improved test suite - tests should run in about 1 second
* update documentation to use positional-only syntax in docstrings
* update readme to pass Python 3 doctest
* add utils module to examples

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* fix readme to pass `twine check`

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* fix bitarrays beings created from unicode in Python 2
* use `PyBytes_*` in C code, treating the Py3k function names as default,
which also removes all redefinitions of `PyString_*`
* handle negative arguments of .index() method consistently with how
they are treated for lists
* add a few more comments to the C code
* move imports outside tests: pickle, io, etc.
* drop Python 2.5 support

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* refactor resize() - only shrink allocated memory if new size falls
lower than half the allocated size
* improve error message when trying to initialize from float or complex

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* fix to compile on Windows with VS 2015, issue 72

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* fix types to actually be types, 29
* check for ambiguous prefix codes when building binary tree for decoding
* remove Python level methods: encode, decode, iterdecode (in favor of
having these implemented on the C-level along with check_codedict)
* fix self tests for Python 2.5 and 2.6
* move all Huffman code related example code into examples/huffman
* add code to generate graphviz .dot file of Huffman tree to examples