Switch on "new" buffer API

[shadchin]

"Old" buffer API will removed in Python 3.10. This is also fix #7930.

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[shadchin]

Can anyone watch this PR?

[shadchin]

I fixed warning with deprecation PyEval_CallObject for Python 3.9

@acozzette Can you rerun tests, please?

[shadchin]

Free bonus from switching on "new" buffer API:

• add support memoryview for Python 2.7

@acozzette It looks like another restart is needed :)

[shadchin]

@acozzette, I don't have access for Kokoro build failed.

Could you, please, help me to understand failure reasons?

[acozzette]

Here is the error message and the last few log lines before it:

py27-cpp run-test: commands[1] | python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++
In file included from /tmp/protobuf/protobuf/python/.tox/py27-cpp/include/python2.7/Python.h:8,
                 from google/protobuf/pyext/descriptor_pool.cc:35:
/tmp/protobuf/protobuf/python/.tox/py27-cpp/include/python2.7/pyconfig.h:1223: error: "_POSIX_C_SOURCE" redefined [-Werror]
 #define _POSIX_C_SOURCE 200112L

In file included from /usr/include/x86_64-linux-gnu/c++/8/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/8/bits/c++config.h:508,
                 from /usr/include/c++/8/type_traits:38,
                 from /usr/include/c++/8/unordered_map:38,
                 from google/protobuf/pyext/descriptor_pool.cc:33:
/usr/include/features.h:265: note: this is the location of the previous definition
 # define _POSIX_C_SOURCE 200809L

In file included from /tmp/protobuf/protobuf/python/.tox/py27-cpp/include/python2.7/Python.h:8,
                 from google/protobuf/pyext/descriptor_pool.cc:35:
/tmp/protobuf/protobuf/python/.tox/py27-cpp/include/python2.7/pyconfig.h:1245: error: "_XOPEN_SOURCE" redefined [-Werror]
 #define _XOPEN_SOURCE 600

In file included from /usr/include/x86_64-linux-gnu/c++/8/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/8/bits/c++config.h:508,
                 from /usr/include/c++/8/type_traits:38,
                 from /usr/include/c++/8/unordered_map:38,
                 from google/protobuf/pyext/descriptor_pool.cc:33:
/usr/include/features.h:202: note: this is the location of the previous definition
 # define _XOPEN_SOURCE 700

cc1plus: all warnings being treated as errors
error: command 'gcc' failed with exit status 1
ERROR: InvocationError for command /tmp/protobuf/protobuf/python/.tox/py27-cpp/bin/python setup.py -q build --cpp_implementation --warnings_as_errors --compile_static_extension (exited with code 1)

[shadchin]

These errors are not related to the change in this PR. It looks like something has changed in the build since October 1, 2020 #7927

I removed --warnings_as_errors for Python 2.7

@acozzette Can you rerun tests, please?