python: Fix package requirements with old setuptools

python: Fix package requirements with old setuptools

Commit 00fcc832d598 ("Update Python package requirements") added a
PEP 508 environment marker to install pywin32 on Windows systems.

This requires a new setuptools version (>= 20.5), but (at least)
RHEL/CentOS7 and Debian Jessie are using an older version of
setuptools and so python extension failed to build.

This commit adds "extras_require" instead of the PEP 508 environment
markers in order to have the conditional dependency of pywin32, but by
remaining compatible with the old setuptools versions.

CC: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
CC: Lucian Petrut <lpetrut@cloudbasesolutions.com>
Fixes: 00fcc832d598 ("Update Python package requirements")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>