Bitarray

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* add official Python 3.9 support
* improve many docstrings
* add DeprecationWarning for `bitdiff()`
* add DeprecationWarning when trying to extend bitarrays
from bytes on Python 3 (`bitarray(b'011')` and `.extend(b'110')`)
* C-level:
- Rewrote `.fromfile()` and `.tofile()` implementation,
such that now the same code is used for Python 2 and 3.
The new implementation is more memory efficient on
Python 3.
- use `memcmp()` in `richcompare()` to shortcut EQ/NE, when
comparing two very large bitarrays for equality the
speedup can easily be 100x
- simplify how unpacking is handled
* add more tests

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* add `.clear()` method (Python 3.3 added this method to lists)
* avoid over-allocation when bitarray objects are initially created
* raise BufferError when resizing bitarrays which is exporting buffers
* add example to study the resize() function
* improve some error messages
* add more tests
* raise `NotImplementedError` with (useful message) when trying to call
the `.fromstring()` or `.tostring()` methods, which have been removed
in the last release

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* add `bitarray.util.make_endian()`
* `util.ba2hex()` and `util.hex2ba()` now also support little-endian
* add `bitarray.get_default_endian()`
* made first argument of initializer a positional-only parameter
* remove `.fromstring()` and `.tostring()` methods, these have been
deprecated 8 years ago, since version 0.4.0
* add `__all__` in `bitarray/__init__.py`
* drop Python 3.3 and 3.4 support

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* `util.ba2hex()` now always return a string object (instead of bytes
object for Python 3), see issue 94
* `util.hex2ba` allows a unicode object as input on Python 2
* Determine 64-bitness of interpreter in a cross-platform fashion 91,
in order to better support PyPy

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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