Fix string formatting to make black --experimental-string-processing happy

- Also add `--experimental-string-processing` to CI

Author: cooperlees

.github/workflows/ci.yml

@@ -34,4 +34,4 @@ jobs:
 
     - name: Check formatting
       run: |
-        black --check .
+        black --check --experimental-string-processing .

bugbear.py

@@ -793,9 +793,10 @@ def visit(self, node):
 
 B904 = Error(
     message=(
-        "B904 Within an `except` clause, raise exceptions with `raise ... from err` "
-        "or `raise ... from None` to distinguish them from errors in exception handling.  "
-        "See https://docs.python.org/3/tutorial/errors.html#exception-chaining for details."
+        "B904 Within an `except` clause, raise exceptions with `raise ... from err` or"
+        " `raise ... from None` to distinguish them from errors in exception handling. "
+        " See https://docs.python.org/3/tutorial/errors.html#exception-chaining for"
+        " details."
     )
 )