Cap protobuf < 4.x

tooz doesn't use upper-constraints so we get the newest version of all
dependencies. It seems one of these, etcd3, is incompatible with recent
versions of protobuf.

  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  If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc 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).

The etcd3 dependency is only used for the etcd3 driver which was
deprecated in change Iff0cd3b51cfc45ecbddc831c55267e80e9c79cac. The
protobuf dependency is only used for etcd3. We can't outright remove the
etcd3 driver yet as it hasn't been long enough since we deprecated it.
That will happen soon enough though and until then we can simply cap
protobuf to an older version that is compatible with the most recent
etcd3 release.

While we're here, we also sync the lower boundaries in the docs
requirments and add some comments indicating where they come from/what
they're for.

Change-Id: I8d6647118be22b0ce55e01b7e5451612ebe30e73
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
(cherry picked from commit e3de0e634a)

doc/requirements.txt

@@ -3,12 +3,27 @@ openstackdocstheme>=2.2.1 # Apache-2.0
 reno>=3.1.0 # Apache-2.0
 
 # Install dependencies for tooz so that autodoc works.
-python-consul>=0.4.7 # MIT License
-sysv-ipc>=0.6.8 # BSD License
+## consul
+python-consul2>=0.0.16 # MIT License
+## etcd
+requests>=2.10.0 # Apache-2.0
+## etcd3
+etcd3>=0.12.0 # Apache-2.0
+grpcio>=1.18.0
+protobuf<4.0 # BSD License (3 clause)
+## etcd3gw
+etcd3gw!=0.2.6,>=0.1.0 # Apache-2.0
+## zake
 zake>=0.1.6 # Apache-2.0
-redis>=2.10.0 # MIT
+## redis
+redis>=3.1.0 # MIT
+## postgresql
 psycopg2>=2.5 # LGPL/ZPL
+## mysql
 PyMySQL>=0.6.2 # MIT License
+## zookeeper
+kazoo>=2.2 # Apache-2.0
+## memcached
 pymemcache!=1.3.0,>=1.2.9 # Apache 2.0 License
-etcd3>=0.6.2 # Apache-2.0
-etcd3gw>=0.1.0 # Apache-2.0
+## ipc
+sysv-ipc>=0.6.8 # BSD License

setup.cfg

@@ -51,6 +51,7 @@ etcd =
 etcd3 =
   etcd3>=0.12.0 # Apache-2.0
   grpcio>=1.18.0
+  protobuf<4.0 # BSD License (3 clause)
 etcd3gw =
   etcd3gw!=0.2.6,>=0.1.0 # Apache-2.0
 zake =