(core) Update logging in sandbox code, and log tracebacks as single log messages.

Summary:
- Replace logger module by the standard module 'logging'.
- When a log message from the sandbox includes newlines (e.g. for tracebacks),
  keep those lines together in the Node log message.

  Previously each line was a different message, making it difficult to view
  tracebacks, particularly in prod where each line becomes a separate message
  object.

- Fix assorted lint errors.

Test Plan: Added a test for the log-line splitting and escaping logic.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3956

sandbox/grist/test_renames2.py

@@ -1,11 +1,11 @@
 import textwrap
 import unittest
 
-import logger
+import logging
 import six
 import test_engine
 
-log = logger.Logger(__name__, logger.INFO)
+log = logging.getLogger(__name__)
 
 
 def _replace_col_name(data, old_name, new_name):