fail-on-template-vars: modernize stack inspection code

[xavfernandez]

inspect.stack() returns a list of namedtuple (or retrocompatible objects) since Python 3.5+: let's use the named attribute.

cf https://docs.python.org/3/library/inspect.html#inspect.stack

And once we have access to a FrameInfo object/namedtuple, access to its frame object and its f_locals member should not need to iterate on all its members: https://docs.python.org/3/reference/datamodel.html#frame-objects