set implicit optional to true in mypy config to avert side effects and test fails from new mypy version

esmvalcore/_task.py

@@ -13,10 +13,8 @@
 import time
 from copy import deepcopy
 from multiprocessing import Pool
-from multiprocessing.pool import ApplyResult
 from pathlib import Path, PosixPath
 from shutil import which
-from typing import Dict, Type
 
 import psutil
 import yaml
@@ -736,7 +734,7 @@ def _run_sequential(self) -> None:
     def _run_parallel(self, max_parallel_tasks=None):
         """Run tasks in parallel."""
         scheduled = self.flatten()
-        running: Dict[Type[BaseTask], Type[ApplyResult]] = {}
+        running = {}
 
         n_tasks = n_scheduled = len(scheduled)
         n_running = 0

setup.cfg

@@ -33,6 +33,8 @@ multi_line_output = 3
 include_trailing_comma = true
 
 [mypy]
+# see mypy.readthedocs.io/en/stable/command_line.html
+implicit_optional = True
 ignore_missing_imports = True
 files = esmvalcore, tests