Bitarray

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* simplify markdown of readme so PyPI renders better
* make tests for bitarray.util required (instead of warning when
they cannot be imported)

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* add bitarray.util module which provides useful utility functions
* deprecate `bitarray.bitdiff()` in favor of `bitarray.util.count_xor()`
* use markdown for documentation
* fix bug in `.count()` on 32bit systems in special cases when array size
is 2^29 bits or larger
* simplified tests by using bytes syntax
* update smallints and sieve example to use new utility module
* simplified mandel example to use numba
* use file context managers in tests

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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