PTS-bot: freeze protobuf python package

4.21.0 version is introducing breaking changes, so freeze it to
3.20.1 for now.

Test: atest pts-bot
Bug: 241962982
Ignore-AOSP-First: Cherry-picked from AOSP
Merged-In: Ie151b67b7340c8a380ee5b80afad5d0b61b22283
Change-Id: Ie151b67b7340c8a380ee5b80afad5d0b61b22283