(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/testutil.py

@@ -6,18 +6,6 @@ import re
 import six
 
 import actions
-import logger
-
-def limit_log_stderr(min_level):
-  """
-  Returns a log handler suitable for logger.set_handler(), which logs using log_stderr but only
-  messages at the given level or higher.
-  """
-  def handler(level, name, msg):
-    if level >= min_level:
-      logger.log_stderr(level, name, msg)
-  return handler
-
 
 def table_data_from_rows(table_id, col_names, rows):
   """