Add requirements.txt for Python Pip

“Requirements files” are files containing a list of items
to be installed using pip install. We're going to need
this for SNYK scans. We can also later use this file
in our pkgdep.sh installation script.

Change-Id: I3224f21ee4cbe170c9840aac38156eeee627c257
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4532


Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Monica Kenguva <monica.kenguva@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>