Compiler warnings when building for Python 3.7 and 3.8

[dlj-NaN]

Example from tox logs:

py37-cpp run-test: commands[1] | python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension
google/protobuf/pyext/message.cc: In function ‘PyObject* google::protobuf::python::cmessage::MergeFromString(google::protobuf::python::CMessage*, PyObject*)’:
google/protobuf/pyext/message.cc:1945:53: error: ‘int PyObject_AsReadBuffer(PyObject*, const void**, Py_ssize_t*)’ is deprecated [-Werror=deprecated-declarations]
   if (PyObject_AsReadBuffer(arg, &data, &data_length) < 0) {
                                                     ^
In file included from /usr/local/include/python3.7m/Python.h:147,
                 from ./google/protobuf/pyext/message.h:37,
                 from google/protobuf/pyext/message.cc:34:
/usr/local/include/python3.7m/abstract.h:489:17: note: declared here
 PyAPI_FUNC(int) PyObject_AsReadBuffer(PyObject *obj,
                 ^~~~~~~~~~~~~~~~~~~~~
google/protobuf/pyext/message.cc:1945:53: error: ‘int PyObject_AsReadBuffer(PyObject*, const void**, Py_ssize_t*)’ is deprecated [-Werror=deprecated-declarations]
   if (PyObject_AsReadBuffer(arg, &data, &data_length) < 0) {
                                                     ^
In file included from /usr/local/include/python3.7m/Python.h:147,
                 from ./google/protobuf/pyext/message.h:37,
                 from google/protobuf/pyext/message.cc:34:
/usr/local/include/python3.7m/abstract.h:489:17: note: declared here
 PyAPI_FUNC(int) PyObject_AsReadBuffer(PyObject *obj,
                 ^~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
error: command 'gcc' failed with exit status 1
ERROR: InvocationError for command /tmp/protobuf/protobuf/python/.tox/py37-cpp/bin/python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension (exited with code 1)
___________________________________ summary ____________________________________
ERROR:   py37-cpp: commands failed

The "new" buffer API is available across all versions we care about, but it will take a little bit of legwork. For now, I think we may want to disable the --warnings_as_errors flag temporarily in our tox config.

[wchargin]

This also shows up in downstream Bazel projects that set
--define=use_fast_cpp_protos=true. It causes spam in all of our build
logs, and there’s nothing that we can do about it. Please fix.

(protobuf is currently the cause of all spam in our build logs; among
all our dependencies, it is uniquely poorly behaved.)

[dlj-NaN]

@wchargin It looks like the corresponding flag has been in BUILD for quite some time now: https://github.com/protocolbuffers/protobuf/blame/master/BUILD#L88

Can you verify what version your WORKSPACE uses for the protobuf library, and what compiler you're using?