python/Makefile: Use python3 while fetching purelib

Some environments (e.g. container images) don't provide either the
"python" binary or "python" symlink - it's more common to always see
a specific version being available in the path, e.g. python3.

Use that to avoid spam like:
  # make clean -j
  /bin/sh: line 1: python: command not found

This also aligns it with other parts of the Makefile (i.e. setup_cmd).

Change-Id: I460c36d8b41bfb13c004ddf4e6b5a1f89b6abfbe
Signed-off-by: Michal Berger <michal.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/20031


Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>