[2.1.x] Restored django.test.utils.patch_logger() for backwards compatibility.

Added back after 607970f31c.

Backport of 8dcd43ce06 from master

django/test/utils.py

@@ -634,6 +634,29 @@ class ignore_warnings(TestContextDecorator):
         self.catch_warnings.__exit__(*sys.exc_info())
 
 
+@contextmanager
+def patch_logger(logger_name, log_level, log_kwargs=False):
+    """
+    Context manager that takes a named logger and the logging level
+    and provides a simple mock-like list of messages received.
+
+    Use unitttest.assertLogs() if you only need Python 3 support. This
+    private API will be removed after Python 2 EOL in 2020 (#27753).
+    """
+    calls = []
+
+    def replacement(msg, *args, **kwargs):
+        call = msg % args
+        calls.append((call, kwargs) if log_kwargs else call)
+    logger = logging.getLogger(logger_name)
+    orig = getattr(logger, log_level)
+    setattr(logger, log_level, replacement)
+    try:
+        yield calls
+    finally:
+        setattr(logger, log_level, orig)
+
+
 # On OSes that don't provide tzset (Windows), we can't set the timezone
 # in which the program runs. As a consequence, we must skip tests that
 # don't enforce a specific timezone (with timezone.override or equivalent),