fix protobuf version to 3.20.0 (#6612)

The python pip deps resolver is not stable, I am fixing the protobuf version to 3.20.0 to avoid conflicts.

Author: pyu10055

tfjs-converter/python/BUILD

@@ -48,6 +48,7 @@ py_wheel(
     license = "Apache 2.0",
     python_tag = "py3",
     requires = [
+        "protobuf==3.20.0",
         "tensorflow>=2.1.0,<3",
         "six>=1.12.0,<2",
         "tensorflow-hub>=0.7.0,<0.13",

tfjs-converter/python/requirements.txt

@@ -1,3 +1,4 @@
+protobuf==3.20.0
 tensorflow>=2.1.0,<3
 numpy~=1.19.2
 six>=1.12.0,<2