docs: def itersheets(self):
          """
          Iterates over the worksheets in the book, and sets the active
          worksheet as the current one before yielding.
          """
          for ws in self.worksheets:
              # Expression with no explicit table specified will use None
              # when calling get_table, which should return the current worksheet/table
              prev_ws = self.active_worksheet
              self.active_worksheet = ws
              try:
                  yield ws
              finally:
                  self.active_worksheet = prev_ws