compat: correctly force a module import in the strip backcompat code

Just calling strip without accessing anything inside the module doesn't
actually trigger the ImportError on older versions if demandimport is active,
so they end up setting debugstrip as the command to wrap, and then dying with
"UnknownCommand: ('debugstrip', [...etc])".
See https://bugzilla.mozilla.org/show_bug.cgi?id=1689647

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branch : stable
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