Unlike ctypes, Cython by default do not release GIL when
calling into C API functions. This causes problems when the
function is long running. As the particular calling thread will
block other python threads by holding the GIL.
This PR explicitly releases GIL when calling into possible
long running functions. It fixes the timeout issue in
PopenPool which previously relied on another python thread
for timeout.
Added a regression test-case by changing sleep to sleep
in FFI, which previously will indefinitely block the popen tests.
Tianqi Chen
2e1666d386