(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_relabeling.py

@@ -1,11 +1,10 @@
+import unittest
+import sys
 import relabeling
 
 from sortedcontainers import SortedListWithKey
 from six.moves import zip as izip, xrange
 
-import unittest
-import sys
-
 # Shortcut to keep code more concise.
 r = relabeling