(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/identifiers.py

@@ -3,6 +3,7 @@
 A module for creating and sanitizing identifiers
 """
 import itertools
+import logging
 import re
 import unicodedata
 from keyword import iskeyword
@@ -10,9 +11,7 @@ from string import ascii_uppercase
 
 import six
 
-import logger
-
-log = logger.Logger(__name__, logger.INFO)
+log = logging.getLogger(__name__)
 
 _invalid_ident_char_re = re.compile(r'[^a-zA-Z0-9_]+')
 _invalid_ident_start_re = re.compile(r'^(?=[0-9_])')