Issue 16998: Clarify that += on a shared value is not atomic.
Fix handling of SystemExit and exit code.  Patch by Brodie Rao.
Issue #19599: Increase sleep period.
Fix indentation in doc example.
Issue #19603: Use specific asserts in test_decr.
Fix compilation error under gcc of the ctypes module bundled libffi for arm.
A variable was declared below the top of a block and one function was using
a K&R C style function declaration!
Fix test.support.bind_port() to not cause an error when Python was compiled
on a system with SO_REUSEPORT defined in the headers but run on a system
with an OS kernel that does not support that reasonably new socket option.