Revert "Use io.open() instead of open()"
Revert "Use io.open() instead of open()"

This reverts commit ef69d5f53ee5503419b33a12b590d1fc8319e152.

Because with io.open the input for lines.setter & content.setter
needs to unicode only.

DBuildService/dockerfile-parse,jpopelka/dockerfile-parse,DBuildService/dockerfile-parse,jpopelka/dockerfile-parse