python: add optional constraint parameter to JobList

Problem: There is no way to pass extra constraints via the JobList
interface.

Add an optional constraint parameter to the JobList initializer. This
parameter takes a string in constraint query syntax which is parsed
by an instance of JobListConstraintParser. The result is then joined
with ``and`` to any other constraints before being passed to the
job-list module.

Author: grondo

src/bindings/python/flux/job/list.py

@@ -220,6 +220,10 @@ class JobList:
     :since: Limit jobs to those that have been active since a given timestamp.
     :name: Limit jobs to those with a specific name.
     :queue: Limit jobs to those submitted to a specific queue or queues
+    :constraint: An RFC 31 Constraint object describing a job-list constraint
+        as documented in RFC 43 Constraint Operators section. This constraint
+        may then be joined with other constraints provided by above parameters
+        via the ``and`` operator.
     """
 
     # pylint: disable=too-many-instance-attributes
@@ -253,6 +257,7 @@ def __init__(
         since=0.0,
         name=None,
         queue=None,
+        constraint=None,
     ):
         self.handle = flux_handle
         self.attrs = list(attrs)
@@ -268,6 +273,7 @@ def __init__(
             for x in fname.split(","):
                 self.add_filter(x)
         self.set_user(user)
+        self.constraint = constraint
 
     def set_user(self, user):
         """Only return jobs for user (may be a username or userid)"""
@@ -319,6 +325,7 @@ def fetch_jobs(self):
             since=self.since,
             name=self.name,
             queue=self.queue,
+            constraint=self.constraint,
         )
 
     def jobs(self):