bpo-38858: Fix ref leak in pycore_interp_init()

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bpo-38858, bpo-38997: _PySys_Create() returns a strong reference to
the sys module: Py_DECREF() is needed when we are done with the
module.

https://bugs.python.org/issue38858

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I tested manually that this change fix https://bugs.python.org/issue38997:

$ ./python -m test -R 3:3 -j0  test__xxsubinterpreters test_atexit test_capi test_threading
(...)
All 4 tests OK.

Total duration: 1 min 2 sec
Tests result: SUCCESS

This bug is not a regression: it exists at least since Python 2.7. It's just that previously, the bug was hidden because subinterpreters were not well isolated from the main interpreter.

@ericsnowcurrently: Welcome to the hell of correctness enforced by subinterpreters :-)

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If this change is merged, PR #17509 can be rejected.