Decouple from PyFilesystem2 dependency

pyfatfs/FatIO.py

@@ -6,18 +6,25 @@
 import threading
 from typing import Union, Optional
 
-from fs.mode import Mode
 from pyfatfs import PyFATException
 
 from pyfatfs.PyFat import PyFat
 
 
+class DefaultModeReadOnly:
+    def __init__(self):
+        self.truncate = False
+        self.appending = False
+        self.reading = True
+        self.writing = False
+
+
 class FatIO(io.RawIOBase):
     """Wrap basic I/O operations for PyFat."""
 
     def __init__(self, fs: PyFat,
                  path: str,
-                 mode: Mode = Mode('r')) -> None:
+                 mode = DefaultModeReadOnly()) -> None:
         """Wrap basic I/O operations for PyFat. **Currently read-only**.
 
         :param fs: `PyFat`: Instance of opened filesystem