don't limit protobuf version to 3

Index: setup.py
--- setup.py.orig
+++ setup.py
@@ -27,7 +27,7 @@ install_requires = [
 install_extras = {
     'client': [
         'gevent>=1.3.0',
-        'protobuf~=3.0; python_version >= "3"',
+        'protobuf; python_version >= "3"',
         'protobuf<3.18.0; python_version < "3"',
         'gevent-eventemitter~=2.1',
     ],
