PR59161 make pretty printers always return strings

PR59161 make pretty printers always return strings

	PR libstdc++/59161
	* python/libstdcxx/v6/printers.py (StdListIteratorPrinter.to_string)
	(StdSlistIteratorPrinter.to_string, StdVectorIteratorPrinter.to_string)
	(StdRbtreeIteratorPrinter.to_string, StdDequeIteratorPrinter.to_string)
	(StdDebugIteratorPrinter.to_string): Return string instead of
	gdb.Value.
	* testsuite/libstdc++-prettyprinters/59161.cc: New test.

From-SVN: r243690