bpo-41123: Remove Py_UNICODE_COPY() and Py_UNICODE_FILL()

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https://bugs.python.org/issue41123

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These functions are not listed in these PEPs:

• https://www.python.org/dev/peps/pep-0623/
• https://www.python.org/dev/peps/pep-0624/

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Py_UNICODE_COPY() and Py_UNICODE_FILL() are no longer used in the PyPI top 5000 projects:

$ ./search_pypi_top_5000.sh Py_UNICODE_COPY
pypi-top-5000_2021-08-17/pipenv-2021.5.29.tar.gz
pypi-top-5000_2021-08-17/Genshi-0.7.5.tar.gz
pypi-top-5000_2021-08-17/frozendict-2.0.6.tar.gz

• pipenv-2021.5.29.tar.gz: pipenv/vendor/markupsafe/_speedups.c is not affected, Py_UNICODE_COPY() is only used on Python 2.7
• Genshi-0.7.5.tar.gz is not affected, Py_UNICODE_COPY() is only used on Python 3.2 and older
• frozendict-2.0.6.tar.gz is not affected, it contains raw copies of unicodeobject.h header file, it doesn't call Py_UNICODE_COPY().

$ ./search_pypi_top_5000.sh Py_UNICODE_FILL
pypi-top-5000_2021-08-17/frozendict-2.0.6.tar.gz

• frozendict-2.0.6.tar.gz is not affected, it contains raw copies of unicodeobject.h header file, it doesn't call Py_UNICODE_FILL().

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cc @methane