libstdc++: Use Python "not in" operator

flake8 warns about code like

    not something in "whatever"

Ordinarily in Python this should be written as:

    something not in "whatever"

This patch makes this change.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (Printer.add_version)
	(add_one_template_type_printer)
	(FilteringTypePrinter.add_one_type_printer): Use Python
	"not in" operator.