etcd3 package incompatible with protobuf 4.x

This isn't so much an issue with tooz as with the etcd3 dependency. The etcd3-based tests are currently failing with the following error message:

    Failed to import test module: tooz.tests.drivers.test_etcd3
    Traceback (most recent call last):
      File "/usr/lib/python3.7/unittest/loader.py", line 436, in _find_test_path
        module = self._get_module_from_name(name)
      File "/usr/lib/python3.7/unittest/loader.py", line 377, in _get_module_from_name
        __import__(name)
      File "/home/zuul/src/opendev.org/openstack/tooz/tooz/tests/drivers/test_etcd3.py", line 22, in <module>
        import tooz.drivers.etcd3 as etcd3_driver
      File "/home/zuul/src/opendev.org/openstack/tooz/tooz/drivers/etcd3.py", line 18, in <module>
        import etcd3
      File "/home/zuul/src/opendev.org/openstack/tooz/.tox/py37/lib/python3.7/site-packages/etcd3/__init__.py", line 3, in <module>
        import etcd3.etcdrpc as etcdrpc
      File "/home/zuul/src/opendev.org/openstack/tooz/.tox/py37/lib/python3.7/site-packages/etcd3/etcdrpc/__init__.py", line 1, in <module>
        from .rpc_pb2 import *
      File "/home/zuul/src/opendev.org/openstack/tooz/.tox/py37/lib/python3.7/site-packages/etcd3/etcdrpc/rpc_pb2.py", line 16, in <module>
        from etcd3.etcdrpc import kv_pb2 as kv__pb2
      File "/home/zuul/src/opendev.org/openstack/tooz/.tox/py37/lib/python3.7/site-packages/etcd3/etcdrpc/kv_pb2.py", line 36, in <module>
        type=None),
      File "/home/zuul/src/opendev.org/openstack/tooz/.tox/py37/lib/python3.7/site-packages/google/protobuf/descriptor.py", line 755, in __new__
        _message.Message._CheckCalledFromGeneratedFile()
    TypeError: Descriptors cannot not be created directly.
    If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
    If you cannot immediately regenerate your protos, some other possible workarounds are:
     1. Downgrade the protobuf package to 3.20.x or lower.
     2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).

Clearly something needs to be fixed in etcd3 itself but etcd3 is unmaintained [1] and the driver itself has been deprecated in tooz [2][3]. We probably want to cap protobuf<4.0 until we can finally remove this driver.

[1] https://github.com/kragniz/python-etcd3/issues/1719
[2] http://lists.openstack.org/pipermail/openstack-discuss/2022-March/027664.html
[3] https://review.opendev.org/c/openstack/tooz/+/833107