On Windows, several commonly available system tools for decompression
are unreliable (gz/bz2/xz). This commit refactors `decompressor_for`
to call out to a Windows or Unix-specific method:
* The decompressor_for_nix method behaves the same as before and
generally treats the Python/system support options for decompression
as interchangeable (although avoids using Python's built-in tar
support since that has had issues with permissions).
* The decompressor_for_win method can only use Python support for
gz/bz2/xz, although for a tar.gz it does use system support for
untar (after the decompression step). .zip uses the system tar
utility, and .Z depends on external support (i.e. that the user
has installed 7zip).
A naming scheme has been introduced for the various _decompression
methods:
* _system_gunzip means to use a system tool (and fail if it's not
available)
* _py_gunzip means to use Python's built-in support for decompressing
.gzip files (and fail if it's not available)
* _gunzip is a method that can do either
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