Fix ftp test_ls_root_dircache() by restoring internal variable after usage to prevent raised exception in __del__ (#1644)

Author: Astropilot

fsspec/implementations/tests/test_ftp.py

@@ -52,12 +52,15 @@ def test_ls_root_dircache(ftp):
 
     assert "/" in fs.dircache
 
+    ftp_tmp = fs.ftp
     fs.ftp = None  # Ensure no ftp action will be done after
 
     files2 = fs.ls("/", detail=False)
 
     assert files == files2
 
+    fs.ftp = ftp_tmp
+
 
 @pytest.mark.parametrize("cache_type", ["bytes", "mmap"])
 def test_complex(ftp_writable, cache_type):