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.