Log Message:
py-protobuf: updated to 3.12.1

Protocol Buffers v3.12.0

Python
[experimental] Added proto3 presence support.
[experimental] fast import protobuf module, only works with cpp generated code \ 
linked in.
Truncate 'float' fields to 4 bytes of precision in setters for pure-Python
implementation (C++ extension was already doing this).
Fixed a memory leak in C++ bindings.
Added a deprecation warning when code tries to create Descriptor objects
directly.
Fix unintended comparison between bytes and string in descriptor.py.
Avoid printing excess digits for float fields in TextFormat.
Remove Python 2.5 syntax compatibility from the proto compiler generated _pb2.py \ 
module code.
Drop 3.3, 3.4 and use single version docker images for all python tests