Move to explicitly creating a new loop

- >= 3.10 add a warning that `get_event_loop` will not automatically create a loop
- Move to explicit API

Test:
- `python3.11 -m venv --upgrade-deps /tmp/tb`
  - `/tmp/tb/bin/pip install -e .`
  - Install deps and no blackd as aiohttp + yarl can't build still with 3.11
  - aio-libs/aiohttp#6600
- `export PYTHONWARNINGS=error`
```
cooper@l33t:~/repos/black$ /tmp/tb/bin/black .
All done! ✨ 🍰 ✨
44 files left unchanged.
```

Fixes #3110

src/black/__init__.py

@@ -773,7 +773,6 @@ def reformat_many(
     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
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903
@@ -788,6 +787,7 @@ def reformat_many(
         # any good due to the Global Interpreter Lock)
         executor = ThreadPoolExecutor(max_workers=1)
 
+    loop = asyncio.new_event_loop()
     try:
         loop.run_until_complete(
             schedule_formatting(