Snakeoil

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- snakeoil.cli.arghparse: Add support for nargs param and accept '0' and '1'
values for the StoreBool action.

- snakeoil.cli.arghparse: Run early parse funcs registered in parent parsers.

- snakeoil.contexts: Add os_environ context manager for os.environ mangling.

- snakeoil.dist.distutils_extensions: Set commands for setuptools by default.

- snakeoil.dist.distutils_extensions: Fully install packages for pytest runs.

- snakeoil.version: Don't display missing extended version message for
releases.

- snakeoil.cli.arghparse: Add support for auto-registering existing
subcommands.

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- snakeoil.dist.iterables: Fix caching_iter sorting when bool is used (57).

- snakeoil.cli.arghparse: Initial subparser support for lazily-imported
subcommand modules using lazy-object-proxy. This allows scripts to split
subcommands into separate modules that are only imported as required.

- snakeoil.dist.distutils_extensions: Force pip to require supported
python versions.

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- snakeoil.formatters: Use simple ANSI term when forcing colors.

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- snakeoil.cli.arghparse: Check for colliding CommaSeparatedElements.

- snakeoil.mappings: Add OrderedFrozenSet implementation which is a
immutable OrderedSet.

- Drop support for python 3.6 and 3.7.

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- snakeoil.cli.arghparse: Make ArgumentParser copyable by default.

- snakeoil.mappings: Add OrderedSet implementation leveraging
guaranteed insertion order for dicts py37 onwards.

- snakeoil.cli.arghparse: Add create_dir argument type that's
similar to the existent_dir type except it creates the dir if
it's missing.

- snakeoil.cli.arghparse: Add ParseNonblockingStdin argparse action
that accepts arguments from stdin in a non-blocking fashion.

- snakeoil.cli.tool: Explicitly handle UserExceptions when parsing args.

- Remove old py2 C extensions.

- snakeoil.dist.distutils_extensions: Force development version
usage when installing from git with pip.

- Add py39 support.

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- snakeoil.cli.arghparse: Add support for registering early parse functions and
rework known arg parsing to allow config file option defaults.