[requirements][tooz] uncap grpcio

Hi all,

Requirements is looking to raise the version of grpcio to 1.21.1. grpcio has been uncapped in requirements for six months now and currently only tooz is blocking the usage of newer versions. Following the unblocking we'd need a release in order to actually unblock things in gate.

It looks like it was fixed in 1.18.0, if you need to mask version between (and including) 1.16.0 and 1.17.x feel free to submit the patches to global-requirements.

https://bugs.launchpad.net/python-tooz/+bug/1808046 70f144abde14e07d7f9620a2abb563ed16ef8c63 (in openstack/tooz)

Thanks,