Update protobuf to at least 3.20

Protobuf 3.20 is not backward-compatible, as explained in
https://github.com/protocolbuffers/protobuf/releases/tag/v3.20.0 .
As commit 01c6639 ("Generate pb2 code with recent protoc") updated
the protobuf files, the dependency also needs to be bumped.

setup.py

@@ -39,7 +39,7 @@ def get_version(rel_path):
         "hidapi",
         "intelhex",
         "Pillow",
-        "protobuf>=3.6",
+        "protobuf>=3.20",
         "requests",
         "tabulate",
     ],