Snakeoil

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- Fix missing requirement files in sdist.

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- Fix ``setup.py develop`` support.

- snakeoil.chksum: Add Whirlpool support.

- Add support for running on PyPy 3.8.

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- snakeoil.dist.distutils_extensions: Migrate to using distutils bundled with
setuptools since distutils is now deprecated in py3.10 and will be removed in
py3.12.

- snakeoil.compression: Simplify registering archive subclasses.

- snakeoil.sequences: Drop namedtuple support -- use the various alternatives
available from the standard library instead.

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- snakeoil.dist.distutils_extensions: Add support for forcing binary wheel
creation.

- snakeoil.osutils: Drop old FsLock related support.

- snakeoil.contexts: Add optional pathspecs param for GitStash.

- snakeoil.dist.distutils_extensions: Drop old OptionalExtension support.

- snakeoil.process.spawn: Drop find_invoking_python() since it's not used by
pkgcore anymore.

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- snakeoil.dist.distutils_extensions: Drop unittest test command support.

- snakeoil.dist.generate_man_rsts: Drop unused script running support.

- snakeoil.cli.arghparse: Drop overly fragile CopyableParser support.

- snakeoil.weakrefs: Drop old WeakValCache support since the related CPython
extension doesn't exist anymore.

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- snakeoil.contents: Add GitStash context manager.

- snakeoil.mappings.OrderedFrozenSet: Support slice notation.

- snakeoil.dist.distutils_extensions: Run pytest as a separate process to
control module search path.

- snakeoil.dist.distutils_extensions: Unconditionally add doc building
commands.