unpin protobuf dependency

setup.py

@@ -7,13 +7,20 @@
 from version import get_version
 from commands import setup_timeseries, build_messages
 
-install_requires = ['six >= 1.8.0', 'basho_erlastic >= 2.1.1', 'protobuf == 3.20.3']
+install_requires = ['six >= 1.8.0', 'basho_erlastic >= 2.1.1']
 requires = ['six(>=1.8.0)', 'basho_erlastic(>= 2.1.1)']
 
 if sys.version_info[:3] <= (2, 7, 9):
     install_requires.append("pyOpenSSL >= 0.14")
     requires.append("pyOpenSSL(>=0.14)")
 
+if sys.version_info[:3] <= (3, 0, 0):
+    install_requires.append('protobuf >=2.4.1, <2.7.0')
+    requires.append('protobuf(>=2.4.1, <2.7.0)')
+else:
+    install_requires.append('protobuf >=3.0.0, <4.0.0')
+    requires.append('protobuf(>=3.0.0, <4.0.0)')
+
 with codecs.open('README.md', 'r', 'utf-8') as f:
     readme_md = f.read()