Lupa

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* fix iteration by returning ``Py_None`` object for ``None`` instead
of ``nil``, which would terminate the iteration

* when converting Python values to Lua, represent ``None`` as a
``Py_None`` object in places where ``nil`` has a special meaning,
but leave it as ``nil`` where it doesn't hurt

* support for counter start value in ``python.enumerate()``

* native implementation for ``python.enumerate()`` that is several
times faster

* much faster Lua iteration over Python objects

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* new helper function ``python.enumerate()`` in Lua that returns a Lua
iterator for a Python object and adds the 0-based index to each
item.

* new helper function ``python.iterex()`` in Lua that returns a Lua
iterator for a Python object and unpacks any tuples that the
iterator yields.

* new helper function ``python.iter()`` in Lua that returns a Lua
iterator for a Python object.

* reestablished the ``python.as_function()`` helper function for Lua
code as it can be needed in cases where Lua cannot determine how to
run a Python function.

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* dropped ``python.as_function()`` helper function for Lua as all
Python objects are callable from Lua now (potentially raising a
``TypeError`` at call time if they are not callable)

* fix regression in 0.13 and later where ordinary Lua functions failed
to print due to an accidentally used meta table

* fix crash when calling ``str()`` on wrapped Lua objects without
metatable

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* support for loading binary Lua modules on systems that support it

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* relicensed to the MIT license used by LuaJIT2 to simplify licensing
considerations

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* fix Cython generated C file using Cython 0.13