py-setuptools_scm: updated to 6.4.2

v6.4.2
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* fix 671 : NoReturn is not avaliable in painfully dead python 3.6

v6.4.1
=======
* fix regression 669: restore get_version signature
* fix 668: harden the selftest for distribution extras

6.4.0
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* compatibility adjustments for setuptools >58
* only put minimal setuptools version into toml extra to warn people with old strict pins
* coorectly handle hg-git self-use
* better mercurial detection
* modernize packaging setup
* python 3.10 support
* better handling of setuptools install command deprecation
* consider ``pyproject.tomls`` when running as command
* use list in git describe command to avoid shell expansions while supporting both windows and posix
* add ``--strip-dev`` flag to ``python -m setuptools_scm`` to print the next guessed version cleanly
* ensure no-guess-dev will fail on bad tags instead of generating invalid versions
* ensure we use utc everywhere to avoid confusion

(adam)