fix PR python/15816

fix PR python/15816

This fixes PR python/15816.

The bug here is that python-selftest.exp can fail:

    No symbol "RETURN_MASK_ALL" in current context.

RETURN_MASK_ALL is a macro, so if macros do not end up in the
debuginfo (very typical) then the test fails.

It seemed simplest to me to simply turn the RETURN_MASK_ defines into
enum constants.  This way they end up in the debuginfo and all is
well.

	PR python/15816:
	* exceptions.h (return_mask): Now an enum.
	(RETURN_MASK_QUIT, RETURN_MASK_ERROR, RETURN_MASK_ALL): Now
	enum constants.

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