Ignore any unused entry that starts with '_'.

This makes it possible to follow PEP8 which recommends '__',
and to name unused entries, e.g.:

    foo, _bar, _zed = func()
fix new hexiom2 benchmark in Py3
minor benchmark tweak
fix explicit <bytes> etc. casts for C++ std::string when auto-decoding is enabled
minor traceback beautification for string conversion helper functions
reduce C code overhead for C++ std::string -> PyObject conversion helpers by implementing them directly in C++ and not Cython
work around PyPy's lack of PyFrozenSet_*() C-API
work around PyPy not supporting PySet_Size() in frozenset objects
undo C++ to-py helper functions rewrite as it lost the ability to extend the set of string-ishly convertible types
improve safety of cname substring replacements for C++ string conversion functions: only replace first occurrence as there may be a user defined portion following
rephrase unhelpful wording in compilation docs
Fix caching issues of cython magic.

Should fix https://github.com/ipython/ipython/issues/6507 when using cython magic. 
We will deprecate cython magic in IPython itself.
Merge pull request #322 from Carreau/patch-1

Fix caching issues of cython magic.
add missing declarations for slice C-API
remove unused import
changelog
extract sys_time.pxd from time.pxd to fix naming collision of "struct timezone" and "long timezone"
split posix time tests in the same way as posix/time.pxd file before
remove unused text from test
make Py_hash_t a known type