Move to explicitly creating a new loop (#3164)

* 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
  - https://github.com/aio-libs/aiohttp/issues/6600
- `export PYTHONWARNINGS=error`
```
cooper@l33t:~/repos/black$ /tmp/tb/bin/black .
All done! ✨ 🍰 ✨
44 files left unchanged.
```

Fixes #3110

* Add to CHANGES.md

* Fix a cooper typo yet again

* Set default asyncio loop to our explicitly created one + unset on exit

* Update CHANGES.md

Fix my silly typo.

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

Co-authored-by: Cooper Ry Lees <me@wcooperlees.com>
Co-authored-by: Thomas Grainger <tagrain@gmail.com>

CHANGES.md

@@ -45,6 +45,9 @@
 
 <!-- Changes to Black's terminal output and error messages -->
 
+- Change from deprecated `asyncio.get_event_loop()` to create our event loop which
+  removes DeprecationWarning (#3164)
+
 ### Packaging
 
 <!-- Changes to how Black is packaged, such as dependency requirements -->

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 +787,8 @@ def reformat_many(
         # any good due to the Global Interpreter Lock)
         executor = ThreadPoolExecutor(max_workers=1)
 
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
     try:
         loop.run_until_complete(
             schedule_formatting(
@@ -801,7 +802,10 @@ def reformat_many(
             )
         )
     finally:
-        shutdown(loop)
+        try:
+            shutdown(loop)
+        finally:
+            asyncio.set_event_loop(None)
         if executor is not None:
             executor.shutdown()