Don't use ?? expression as it's too new

python/ycm/vimsupport.py

@@ -182,7 +182,7 @@ def GetCurrentBufferNumber():
 
 
 def GetBufferChangedTick( bufnr ):
-  return GetIntValue( f'getbufvar({ bufnr }, "changedtick") ?? 0' )
+  return GetIntValue( f'getbufvar({ bufnr }, "changedtick")' ) or 0
 
 
 # Returns a range covering the earliest and latest lines visible in the current