Add shim for cancel_futures in older Pythons

Thanks @hfwittmann

Closes #993

Co-authored-by: H. Felix Wittmann <hfwittmann@users.noreply.github.com>

src/ocrmypdf/builtin_plugins/concurrency.py

@@ -104,6 +104,15 @@ def thread_init(q: Queue, user_init: UserInit, loglevel) -> None:
 class StandardExecutor(Executor):
     """Standard OCRmyPDF concurrent task executor."""
 
+    def _cancel_futures_kwargs(self):
+        """Shim older Pythons that do not have Executor.shutdown(...cancel_futures=).
+
+        Remove this code when support for Python 3.8 is dropped.
+        """
+        if sys.version_info[:2] < (3, 9):
+            return {}
+        return dict(cancel_futures=True)
+
     def _execute(
         self,
         *,
@@ -142,7 +151,7 @@ class StandardExecutor(Executor):
                     task_finished(result, pbar)
             except KeyboardInterrupt:
                 # Terminate pool so we exit instantly
-                executor.shutdown(wait=False, cancel_futures=True)
+                executor.shutdown(wait=False, **self._cancel_futures_kwargs())
                 raise
             except Exception:
                 if not os.environ.get("PYTEST_CURRENT_TEST", ""):
@@ -151,7 +160,7 @@ class StandardExecutor(Executor):
                     # results will be discard. But if the condition above is True,
                     # then we are running in pytest, and we want everything to exit
                     # as cleanly as possible so that we get good error messages.
-                    executor.shutdown(wait=False, cancel_futures=True)
+                    executor.shutdown(wait=False, **self._cancel_futures_kwargs())
                 raise
             finally:
                 # Terminate log listener