[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

cheroot/server.py

@@ -217,7 +217,7 @@ def __call__(self, rfile, hdict=None):  # noqa: C901  # FIXME
                 # NOTE: The latter is equivalent and does not.
                 # It's a continuation line.
                 if k is None:
-                    raise ValueError("Illegal continuation line.")
+                    raise ValueError('Illegal continuation line.')
                 v = line.strip()
             else:
                 try: