Bitarray

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* update pythoncapi_compat: Fix support with PyPy 3.7, 120
* update readme

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* switch documentation from markdown to reStructuredText
* add tests

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* add shift operations (`<<`, `>>`, `<<=`, `>>=`), see 117
* add `bitarray.util.ba2base()` and `bitarray.util.base2ba()`,
see last paragraph in [Bitarray representations](represent.rst)
* documentation and tests

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* fix crash caused by unsupported types in binary operations, 116
* speedup initializing or extending a bitarray from another with different
bit-endianness
* add formatting options to `bitarray.util.pprint()`
* add documentation on [bitarray representations](represent.rst)
* add and improve tests (all 291 tests run in less than half a second on
a modern machine)

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* moved implementation of and `hex2ba()` and `ba2hex()` to C-level
* add `bitarray.util.parity()`

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* add `bitarray.util.serialize()` and `bitarray.util.deserialize()`
* allow whitespace (ignore space and `\n\r\t\v`) in input strings,
e.g. `bitarray('01 11')` or `a += '10 00'`
* add `bitarray.util.pprint()`
* When initializing a bitarray from another with different bit-endianness,
e.g. `a = bitarray('110', 'little')` and `b = bitarray(a, 'big')`,
the buffer used to be simply copied, with consequence that `a == b` would
result in `False`. This is fixed now, that is `a == b` will always
evaluate to `True`.
* add test for loading existing pickle file (created using bitarray 1.5.0)
* add example showing how to [jsonize bitarrays](../examples/extend_json.py)
* add tests