Python 3.x emits `ResourceWarning`s if certains resources aren't correctly
closed. Due to the way Mitogen has been terminating child processes this has
been occurring.
```
test_dev_tty_open_succeeds
(create_child_test.TtyCreateChildTest.test_dev_tty_open_succeeds) ...
/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/subprocess.py:1127:
ResourceWarning: subprocess 3313 is still running
_warn("subprocess %s is still running" % self.pid,
ResourceWarning: Enable tracemalloc to get the object allocation traceback
ok
```
During garbage collection subprocess.Popen() objects emit
ResourceWarning("subprocess 123 is still running")
if proc.returncode hasn't been set. Typically calling proc.wait() does so,
once the sub-process has exited. Calling os.waitpid(proc.pid, 0) also waits
for the sub-process to exit, but it doesn't update proc.returncode, so the
ResourceWarning is still emitted.
This change exposes `subprocess.Popen` methods on
`mitogen.parent.PopenProcess`, so that the returncode can be set.
See https://gist.github.com/moreati/b8d157ff82cb15234bece4033accc5e5
Alex Willmer