master: Fix LogForwarder in case an own LogRecordFactory is used

Since Python 3.2 the log record factory can be changed by using
`logging.setLogRecordFactory` [1]. Therefore use `logging.makeLogRecord` as
recommended in the documentation:

"LogRecord instances are created automatically by the Logger every time
something is logged, and can be created manually via makeLogRecord() (for
example, from a pickled event received over the wire)." [2]

This fixes the test case
`log_handler_test.LogRecordFactoryTest.test_logrecordfactory`.

[1] https://docs.python.org/3/library/logging.html#logging.setLogRecordFactory
[2] https://docs.python.org/3/library/logging.html#logrecord-objects

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>

mitogen/master.py

@@ -370,15 +370,19 @@ class LogForwarder(object):
         if logger is None:
             self._cache[logger_name] = logger = logging.getLogger(logger_name)
 
+        levelno = int(level_s)
         # See logging.Handler.makeRecord()
-        record = logging.LogRecord(
-            name=logger.name,
-            level=int(level_s),
-            pathname='(unknown file)',
-            lineno=0,
-            msg=s,
-            args=(),
-            exc_info=None,
+        record = logging.makeLogRecord(
+            {
+                "name": logger.name,
+                "levelname": logging.getLevelName(levelno),
+                "levelno": levelno,
+                "pathname": "(unknown file)",
+                "lineno": 0,
+                "msg": s,
+                "args": (),
+                "exc_info": None,
+            }
         )
         record.mitogen_message = s
         record.mitogen_context = self._router.context_by_id(msg.src_id)