Correct incorrect type annotation inside strategy code

This was not causing an error in master because no uses of the
incorrectly annotated attribute (self.executors) were type checked.

This PR introduces a typed dict to represent the (single element) dictionary
used for strategy state.

(A futher tidyup could be to remove that typed dict entirely and replace with
the single value it contains, but this PR is trying to be descriptive of
existing code)

The incorrect type annotation was added in #2712

parsl/jobs/strategy.py

@@ -3,12 +3,11 @@
 import time
 import math
 import warnings
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, TypedDict
 
 import parsl.jobs.job_status_poller as jsp
 
 from parsl.executors import HighThroughputExecutor
-from parsl.executors.base import ParslExecutor
 from parsl.executors.status_handling import BlockProviderExecutor
 from parsl.jobs.states import JobState
 from parsl.process_loggers import wrap_with_logs
@@ -17,6 +16,16 @@
 logger = logging.getLogger(__name__)
 
 
+class ExecutorState(TypedDict):
+    """Strategy relevant state for an executor
+    """
+
+    idle_since: Optional[float]
+    """The timestamp at which an executor became idle.
+    If the executor is not idle, then None.
+    """
+
+
 class Strategy:
     """Scaling strategy.
 
@@ -115,7 +124,7 @@ class Strategy:
 
     def __init__(self, *, strategy: Optional[str], max_idletime: float):
         """Initialize strategy."""
-        self.executors: Dict[str, ParslExecutor]
+        self.executors: Dict[str, ExecutorState]
         self.executors = {}
         self.max_idletime = max_idletime