(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/gencode.py

@@ -15,7 +15,7 @@ The schema for grist data is:
     "formula": <opt_string>,
   }
 """
-import linecache
+import logging
 import re
 import imp
 from collections import OrderedDict
@@ -28,8 +28,7 @@ import summary
 import table
 import textbuilder
 from usertypes import get_type_default
-import logger
-log = logger.Logger(__name__, logger.INFO)
+log = logging.getLogger(__name__)
 
 indent_str = "  "