Use ZFS version for pyzfs & drop unused reqs file

Now that 'pyzfs' is part of the ZFS codebase, it should be
versioned the same as the rest of the source tree. This eliminates
confusion on what version of the bindings are being used, especially
for dependent Python projects that may use the Python dist metadata
to identify compatible versions of pyzfs to work from.

In addition, a trivial change to drop the unused requirements.txt
file is included, simply because it's unused and a leftover from
before it was imported into the ZFS codebase and wired into the
autotools build scripts.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Neal Gompa <ngompa@datto.com>
Closes #8243