(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/parse_data.py

@@ -15,6 +15,7 @@ import six
 from six.moves import zip, xrange
 
 log = logging.getLogger(__name__)
+log.setLevel(logging.WARNING)
 
 
 # Typecheck using type(value) instead of isinstance(value, some_type) makes parsing 25% faster