workaround no os.pathconf() on Windows platforms

- Previous PR to truncate long filenames won't work on windows
  due to lack of support for os.pathconf(). This works around the
  limitation by hardcoding the value for PC_NAME_MAX when pathconf
  is unavailable.

Author: lstein

ldm/invoke/dynamic_prompts.py

@@ -116,7 +116,10 @@ def _get_fn_format(directory:str, sequence:int)->str:
     Get a filename that doesn't exceed filename length restrictions
     on the current platform.
     """
-    max_length = os.pathconf(directory,'PC_NAME_MAX')
+    try:
+        max_length = os.pathconf(directory,'PC_NAME_MAX')
+    except:
+        max_length = 255
     prefix = f'dp.{sequence:04}.'
     suffix = '.png'
     max_length -= len(prefix)+len(suffix)