mgr/cephadm: track dependency in tox.ini

mgr/cephadm: track dependency in tox.ini

* track dependency for running tox tests with tox.ini. the fix
  test is not performed by "make check", so `install-deps.sh`
  is not responsible for preparing the autopep8 wheels. hence
  we can just put it in tox.ini
* remove "python_version >= '3'" as we do not support python2

this change addresses a regression introduced by
7d8a3b60321da10af6f4557a9d69eda09ca2c620, which fails the rpmbuild like:

RPM build errors:
    Installed (but unpackaged) file(s) found:
   /usr/share/ceph/mgr/requirements-fix.txt

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit a196672b77bf5f1806e0c1c8b906052cf45a6afe)