Log Message:
protobuf py-protobuf: updated to [34].21.3

Protocol Buffers v21.3

C++

Add header search paths to Protobuf-C++.podspec
Fixed Visual Studio constinit errors
Fix 9947: make the ABI compatible between debug and non-debug builds

UPB

Allow empty package names (fixes behavior regression in 4.21.0)
Fix a SEGV bug when comparing a non-materialized sub-message
Fix several bugs in descriptor mapping containers (eg. descriptor.services_by_name)
for x in mapping now yields keys rather than values, to match Python conventions \ 
and the behavior of the old library.
Lookup operations now correctly reject unhashable types as map keys.
We implement repr() to use the same format as dict.
Fix maps to use the ScalarMapContainer class when appropriate
Fix bug when parsing an unknown value in a proto2 enum extension

PHP

Add "readonly" as a keyword for PHP and add previous classnames to \ 
descriptor pool

Python

Make //:protobuf_python and //:well_known_types_py_pb2 public