Return a StackSummary object representing a list of
pre-processed entries from traceback.
This is useful for alternate formatting of stack traces. If
'limit' is omitted or None, all entries are extracted. A
pre-processed stack trace entry is a FrameSummary object
representing the information that is usually printed for a
stack trace. The line attribute is a string with
leading and trailing whitespace stripped; if the source is not
available the corresponding attribute is None.