Support building .cpp files and linking them into the micropython
executable in a way similar to how it is done for .c files. The main
incentive here is to enable user C modules to use C++ files (which are put
in SRC_MOD_CXX by py.mk) since the core itself does not utilize C++.
However, to verify build functionality a unix overage test is added. The
esp32 port already has CXXFLAGS so just add the user modules' flags to it.
For the unix port use a copy of the CFLAGS but strip the ones which are not
usable for C++.
This adds the Python files in the tests/ directory to be formatted with
./tools/codeformat.py. The basics/ subdirectory is excluded for now so we
aren't changing too much at once.
In a few places `# fmt: off`/`# fmt: on` was used where the code had
special formatting for readability or where the test was actually testing
the specific formatting.
This behaviour of a NULL write C method on a stream that uses the write
adaptor objects is no longer supported. It was only ever used by the
coverage build for testing the fail path of mp_get_stream_raise().
