Use a defaultdict for BlockInfo in htex scale-in (#3300)

This is to accomodate upcoming expansion of the block_info dictionary to
include data from an additional source (the list of blocks known to the
scaling code)

This change arises from a review of PR #3232, an earlier prototype of that
expansion work.

Behaviourally, this shouldn't change anything: it changes invalid
block accesses from key errors into blocks that are infinitely idle and
completely unloaded. However there are no indexes into block_info except
in code that is adding block information, so this situation should
not arise.

parsl/executors/high_throughput/executor.py

@@ -1,4 +1,5 @@
 import typing
+from collections import defaultdict
 from concurrent.futures import Future
 import typeguard
 import logging
@@ -730,13 +731,11 @@ class BlockInfo:
             idle: float  # shortest idle time of any manager in this block
 
         managers = self.connected_managers()
-        block_info: Dict[str, BlockInfo] = {}
+        block_info: Dict[str, BlockInfo] = defaultdict(lambda: BlockInfo(tasks=0, idle=float('inf')))
         for manager in managers:
             if not manager['active']:
                 continue
             b_id = manager['block_id']
-            if b_id not in block_info:
-                block_info[b_id] = BlockInfo(tasks=0, idle=float('inf'))
             block_info[b_id].tasks += manager['tasks']
             block_info[b_id].idle = min(block_info[b_id].idle, manager['idle_duration'])