Avoid "is" with a literal

Eliminates python warnings

Author: cryobry

btrfs-clone

@@ -141,7 +141,7 @@ def get_subvols(mnt):
  svs = []
  for line in vols.decode("ascii").split("\n"):
  # Skip header lines
- if len(line) is 0 or not line[0].isdigit():
+ if len(line) == 0 or not line[0].isdigit():
  continue
  try:
  sv = Subvol(mnt, line.split()[3])