libstdc++: _versioned_namespace is always non-None

libstdc++: _versioned_namespace is always non-None

Some code in the pretty-printers seems to assume that the
_versioned_namespace global might be None (or the empty string).
However, doesn't occur, as the variable is never reassigned.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py: Assume that
	_versioned_namespace is non-None.
	* python/libstdcxx/v6/xmethods.py (is_specialization_of):
	Assume that _versioned_namespace is non-None.

(cherry picked from commit d342c9de6a1534cbce324bcc3c7c0898ff74d386)