protobuf py-protobuf: updated to 3.15.0

Protocol Buffers v3.15.0

Protocol Compiler

Optional fields for proto3 are enabled by default, and no longer require
the --experimental_allow_proto3_optional flag.

C++

MessageDifferencer: fixed bug when using custom ignore with multiple
unknown fields
Use init_seg in MSVC to push initialization to an earlier phase.
Runtime no longer triggers -Wsign-compare warnings.
Fixed -Wtautological-constant-out-of-range-compare warning.
DynamicCastToGenerated works for nullptr input for even if RTTI is disabled
Arena is refactored and optimized.
Clarified/specified that the exact value of Arena::SpaceAllocated() is an
implementation detail users must not rely on. It should not be used in
unit tests.
Change the signature of Any::PackFrom() to return false on error.
Add fast reflection getter API for strings.
Constant initialize the global message instances
Avoid potential for missed wakeup in UnknownFieldSet
Now Proto3 Oneof fields have "has" methods for checking their presence in
C++.
Bugfix for NVCC
Return early in _InternalSerialize for empty maps.
Adding functionality for outputting map key values in proto path logging
output (does not affect comparison logic) and stop printing 'value' in the
path. The modified print functionality is in the
MessageDifferencer::StreamReporter.
Ensure that null char symbol, package and file names do not result in a
crash.
Constant initialize the global message instances
Pretty print 'max' instead of numeric values in reserved ranges.
Removed remaining instances of std::is_pod, which is deprecated in C++20.
Changes to reduce code size for unknown field handling by making uncommon
cases out of line.
Fix std::is_pod deprecated in C++20
Fix some -Wunused-parameter warnings
Fix detecting file as directory on zOS issue 8051
Don't include sys/param.h for _BYTE_ORDER
remove CMAKE_THREAD_LIBS_INIT from pkgconfig CFLAGS
Fix TextFormatMapTest.DynamicMessage issue 5136
Fix for compiler warning issue 8145
fix: support deprecated enums for GCC < 6
Fix some warning when compiling with Visual Studio 2019 on x64 target

Python

Provided an override for the reverse() method that will reverse the internal
collection directly instead of using the other methods of the BaseContainer.
MessageFactory.CreateProtoype can be overridden to customize class creation.
Fix PyUnknownFields memory leak
Add macOS big sur compatibility


(adam)