(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/imports/import_csv.py

@@ -14,6 +14,7 @@ from imports import import_utils
 
 
 log = logging.getLogger(__name__)
+log.setLevel(logging.WARNING)
 
 SCHEMA = [
           {

sandbox/grist/imports/import_xls.py

@@ -4,16 +4,17 @@ and returns a object formatted so that it can be used by grist for a bulk add re
 """
 import logging
 
-import six
 import openpyxl
 from openpyxl.utils.datetime import from_excel
-from openpyxl.worksheet import _reader
+from openpyxl.worksheet import _reader    # pylint:disable=no-name-in-module
+import six
 from six.moves import zip
 
 import parse_data
 from imports import import_utils
 
 log = logging.getLogger(__name__)
+log.setLevel(logging.WARNING)
 
 
 # Some strange Excel files have values that are marked as dates but are invalid as dates.