warnings: subprocpool - unclosed file

[oliver-sanders]

Address a Python warning:

/net/home/h06/osanders/cylc-flow/cylc/flow/subprocpool.py:333: ResourceWarning: unclosed file <_io.BufferedRandom name=18>
  proc = cls._run_command_init(ctx)
ResourceWarning: Enable tracemalloc to get the object allocation traceback

To see this warning, run the tests or try adding:

import warnings
warnings.filterwarnings('once')

Warning spotted part way through a workflow run.