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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.

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.