try-except tomllib import

See #2965 

I left the version check in place because mypy doesn't generally like try-excepted imports.

src/black/files.py

@@ -22,7 +22,11 @@
 from pathspec.patterns.gitwildmatch import GitWildMatchPatternError
 
 if sys.version_info >= (3, 11):
-    import tomllib
+    try:
+        import tomllib
+    except ImportError:
+        # Help users on older alphas
+        import tomli as tomllib
 else:
     import tomli as tomllib