rtutil: Fix crash when multiple RTLinkCaches race.

This was most likely to happen with multiple bean-checks running in
parallel: they would both see a new RT URL, fetch it, and try to
insert it into the cache. That's not a problem as long as they're
both inserting the same URL, so catch that exception.

conservancy_beancount/rtutil.py

@@ -171,10 +171,16 @@ class RTLinkCache(_LinkCache):
             self._nourls.add(key)
         else:
             ticket_id, attachment_id = key
+            try:
                 self._db.execute(
                     'INSERT INTO RTLinkCache VALUES(?, ?, ?)',
                     (ticket_id, attachment_id, value),
                 )
+            except sqlite3.IntegrityError:
+                # Another instance might've inserted the URL before we did.
+                # That's fine as long as it's the same URL, which it should be.
+                if value != self.get(key):
+                    raise
 
     def __delitem__(self, key: TicketAttachmentIds) -> None:
         raise NotImplementedError("RTLinkCache.__delitem__")