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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_request_method(request: web.Request):
 """Parse Access-Control-Request-Method header of the preflight request """

        method = request.headers.get(hdrs.ACCESS_CONTROL_REQUEST_METHOD)
        if method is None:
            raise web.HTTPForbidden(
                text="CORS preflight request failed: "
                     "'Access-Control-Request-Method' header is not specified")

        # FIXME: validate method string (ABNF: method = token), if parsing
        # fails, raise HTTPForbidden.

        return method