BUG, COMPAT: set may not be sorted

Author: mattip

pandas/_libs/parsers.pyx

@@ -454,7 +454,8 @@ cdef class TextReader:
             if callable(usecols):
                 self.usecols = usecols
             else:
-                self.usecols = set(usecols)
+                self.usecols = list(set(usecols))
+                self.usecols.sort()
 
         # XXX
         if skipfooter > 0: