filesystems/py-libzfs: patch collections.Sequence import error

In Python 3.10 collections.Sequence module was moved into the
collections.abc.Sequence module, but nvpair.pxi is still using the old
path.

This patch fixes the import and also keeps backwards compatibility with
<=py3.9.

While I'm here, clean up the Makefile's style, and take maintainership.

PR: 293920
Approved by: Goran Mekić (maintainer)
Sponsored by: ConnectWise
Reported by: js