protobuf package is present in upper-constraints, thus should not be
specified in requirements to version that is smaller that one on u-c,
otherwise pip will fail due to conflict between requirements and
constraints.
We do that only for docs and releasenotes, since used workaround
decreases performance.
The issue is mainly raised by etcd3 module that we rely on, so not
to affect runtime things a left as is for now.
Change-Id: Ieeba8d53a8311ed6cc228f444c831d148ee29025
7d50893625