Use add_color_level() conditionally

According to [1] add_color_level() is a method of ColoredLevelFormatter
only.

[1] https://github.com/pytest-dev/pytest/blob/main/src/_pytest/logging.py#L104

Change-Id: Iaa67c613b196b9cbbbc930d547edaa784d545130

Author: Roman Kuznecov

labgrid/pytestplugin/hooks.py

@@ -43,7 +43,8 @@ def set_cli_log_level(level):
 
 
 def configure_pytest_logging(config, plugin):
-    plugin.log_cli_handler.formatter.add_color_level(logging.CONSOLE, "blue")
+    if hasattr(plugin.log_cli_handler.formatter, "add_color_level"):
+        plugin.log_cli_handler.formatter.add_color_level(logging.CONSOLE, "blue")
     plugin.log_cli_handler.setFormatter(StepFormatter(
         color=config.option.lg_colored_steps,
         parent=plugin.log_cli_handler.formatter,