bindings/python: explicitly clear futures

Problem: job.submit() and job.wait() both seem to
leak futures.

The futures in these "synchronous" methods go out
of scope and thus should be automatically destroyed,
but due to a circular reference alluded to by
@SteVwonder in #2549, they persist.

As a workaround, explicitly call _clear() on the
futures in these methods.

Credit goes to @andre-merzky for proposing the first
version of this patch in #2553, based on a suggestion
by @grondo, with changes proposed by @SteVwonder.
Group effort!

Fixes #2549.

src/bindings/python/flux/job.py

@@ -52,7 +52,10 @@ def submit_get_id(future):
 
 def submit(flux_handle, jobspec, priority=lib.FLUX_JOB_PRIORITY_DEFAULT, flags=0):
     future = submit_async(flux_handle, jobspec, priority, flags)
-    return submit_get_id(future)
+    jid = submit_get_id(future)
+    # pylint: disable=protected-access
+    future.pimpl._clear()
+    return jid
 
 
 def wait_async(flux_handle, jobid=lib.FLUX_JOBID_ANY):
@@ -78,4 +81,7 @@ def wait_get_status(future):
 
 def wait(flux_handle, jobid=lib.FLUX_JOBID_ANY):
     future = wait_async(flux_handle, jobid)
-    return wait_get_status(future)
+    status = wait_get_status(future)
+    # pylint: disable=protected-access
+    future.pimpl._clear()
+    return status