Move imports of ThreadPoolExecutor into reformat_many(), allowing Black-in-the-browser (#3046)

This is a slight perf win for use-cases that don't invoke `reformat_many()`, but more importantly to me today it means I can use Black in pyscript.

src/black/__init__.py

@@ -1,7 +1,6 @@
 import asyncio
 from json.decoder import JSONDecodeError
 import json
-from concurrent.futures import Executor, ThreadPoolExecutor, ProcessPoolExecutor
 from contextlib import contextmanager
 from datetime import datetime
 from enum import Enum
@@ -17,6 +16,7 @@
 import tokenize
 import traceback
 from typing import (
+    TYPE_CHECKING,
     Any,
     Dict,
     Generator,
@@ -77,6 +77,9 @@
 
 from _black_version import version as __version__
 
+if TYPE_CHECKING:
+    from concurrent.futures import Executor
+
 COMPILED = Path(__file__).suffix in (".pyd", ".so")
 
 # types
@@ -767,6 +770,8 @@ def reformat_many(
     workers: Optional[int],
 ) -> None:
     """Reformat multiple files using a ProcessPoolExecutor."""
+    from concurrent.futures import Executor, ThreadPoolExecutor, ProcessPoolExecutor
+
     executor: Executor
     loop = asyncio.get_event_loop()
     worker_count = workers if workers is not None else DEFAULT_WORKERS
@@ -808,7 +813,7 @@ async def schedule_formatting(
     mode: Mode,
     report: "Report",
     loop: asyncio.AbstractEventLoop,
-    executor: Executor,
+    executor: "Executor",
 ) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 

tests/test_black.py

@@ -922,7 +922,7 @@ def err(msg: str, **kwargs: Any) -> None:
         self.assertEqual("".join(err_lines), "")
 
     @event_loop()
-    @patch("black.ProcessPoolExecutor", MagicMock(side_effect=OSError))
+    @patch("concurrent.futures.ProcessPoolExecutor", MagicMock(side_effect=OSError))
     def test_works_in_mono_process_only_environment(self) -> None:
         with cache_dir() as workspace:
             for f in [
@@ -1683,7 +1683,7 @@ def test_cache_single_file_already_cached(self) -> None:
     def test_cache_multiple_files(self) -> None:
         mode = DEFAULT_MODE
         with cache_dir() as workspace, patch(
-            "black.ProcessPoolExecutor", new=ThreadPoolExecutor
+            "concurrent.futures.ProcessPoolExecutor", new=ThreadPoolExecutor
         ):
             one = (workspace / "one.py").resolve()
             with one.open("w") as fobj:
@@ -1792,7 +1792,7 @@ def test_write_cache_creates_directory_if_needed(self) -> None:
     def test_failed_formatting_does_not_get_cached(self) -> None:
         mode = DEFAULT_MODE
         with cache_dir() as workspace, patch(
-            "black.ProcessPoolExecutor", new=ThreadPoolExecutor
+            "concurrent.futures.ProcessPoolExecutor", new=ThreadPoolExecutor
         ):
             failing = (workspace / "failing.py").resolve()
             with failing.open("w") as fobj: