Update black, and run auto formatting over the codebase (#9381)

- Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](80d6dc9783/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

synapse/util/distributor.py

@@ -108,7 +108,10 @@ class Signal:
                 return await maybe_awaitable(observer(*args, **kwargs))
             except Exception as e:
                 logger.warning(
-                    "%s signal observer %s failed: %r", self.name, observer, e,
+                    "%s signal observer %s failed: %r",
+                    self.name,
+                    observer,
+                    e,
                 )
 
         deferreds = [run_in_background(do, o) for o in self.observers]