asyn_wrapper: avoid creating async version of open method (#1769)

The `_open` method is part of an AbstractFileSystem interface, and it is
supposed to be synchronous.

Author: skshetry

fsspec/implementations/asyn_wrapper.py

@@ -57,8 +57,9 @@ def _wrap_all_sync_methods(self):
         """
         Wrap all synchronous methods of the underlying filesystem with asynchronous versions.
         """
+        excluded_methods = {"open"}
         for method_name in dir(self.sync_fs):
-            if method_name.startswith("_"):
+            if method_name.startswith("_") or method_name in excluded_methods:
                 continue
 
             attr = inspect.getattr_static(self.sync_fs, method_name)