[issue19835] Add a MemoryError singleton to fix an unlimited loop when the memory is exhausted

[STINNER Victor]

STINNER Victor added the comment:

> Under very low memory condition, PyErr_NoMemory() or PyErr_NormalizeException() enters an unlimited loop when the free list of MemoryError becomes empty.

I got this bug when I worked on the issue #19817 which adds an arbitary limit to memory allocations using tracemalloc. I used this limit on the Python test suite to check how Python behaves under very low memory condition.

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<http://bugs.python.org/issue19835>
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