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In the latest MSVC in C++17 mode including Eigen causes warnings:

    warning C4996: 'std::unary_negate<_Fn>': warning STL4008: std::not1(),
    std::not2(), std::unary_negate, and std::binary_negate are deprecated in
    C++17. They are superseded by std::not_fn(). You can define
    _SILENCE_CXX17_NEGATORS_DEPRECATION_WARNING or
    _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS to acknowledge that you have
    received this warning.

This disables 4996 for the Eigen includes.

Catch generates a similar warning for std::uncaught_exception, so
disable the warning there, too.

In both cases this is temporary; we can (and should) remove the warnings
disabling once new upstream versions of Eigen and Catch are available
that address the warning. (The Catch one, in particular, looks to be
fixed in upstream master, so will probably be fixed in the next (2.0.2)
release).

Fixes a race condition when multiple threads try to acquire the GIL
before `detail::internals` have been initialized. `gil_scoped_release`
is now tasked with initializing `internals` (guaranteed single-threaded)
to ensure the safety of subsequent `acquire` calls from multiple threads.

Py_Finalize could potentially invoke code that calls `get_internals()`,
which could create a new internals object if one didn't exist.
`finalize_interpreter()` didn't catch this because it only used the
pre-finalize interpreter pointer status; if this happens, it results in
the internals pointer not being properly destroyed with the interpreter,
which leaks, and also causes a `get_internals()` under a future
interpreter to return an internals object that is wrong in various ways.

Fixes #887.

At this point, there is only a single test for interpreter basics.

Apart from embedding itself, having a C++ test framework will also
benefit the C++-side features by allowing them to be tested directly.