bugfix: mismatch in protobuf dep between requirements.txt and setup.py

This is the source of a bug in which this package will break other systems by allowing the incompatible version 4.* of protobuf to be installed.

Author: ZephSibley

setup.py

@@ -20,7 +20,7 @@
     install_requires=[
         "deprecated==1.2.10",
         "cryptography>=2.2.1",
-        "protobuf>=3.1.0",
+        "protobuf==3.13.0",
         "requests>=2.11.1",
         "future>=0.11.0",
         "asn1==2.2.0",