Ensure before_sleep_log message isn't multiline

Some exceptions add extra `\n` characters to their message, causing the last . in the log to be a new line.

Author: sebastian-correa

releasenotes/notes/multline-error-log-fix-f4791dd41c9915a3.yaml

@@ -0,0 +1,5 @@
+fixes:
+  - |
+    Strip the ``str`` representation of the exception to avoid multiline
+    log output from ``before_sleep()`` when some exceptions include ``\n``
+    characters at the end of their string representation.

tenacity/before_sleep.py

@@ -46,7 +46,8 @@ def log_it(retry_state: "RetryCallState") -> None:
 
         if retry_state.outcome.failed:
             ex = retry_state.outcome.exception()
-            verb, value = "raised", f"{ex.__class__.__name__}: {ex}"
+            exception_str = str(ex).strip()  # Ensure message isn't multiline.
+            verb, value = "raised", f"{ex.__class__.__name__}: {exception_str}"
 
             if exc_info:
                 local_exc_info = retry_state.outcome.exception()