TST: numpy_test: run some of the buffer tests only when a sufficiently new Numpy version is available

Small fix was needed to make the buffer string parsing handle this case
correctly.

clean up special casing of non-portable builtin types, add option for disabling errors on unknown names

Several ``literalinclude``s in the User's Guide Tutorial
(``tutorial.rst``) had incorrect paths (off by one parent directory),
causing a failure to include code examples in the built documentation.
This patch corrects those paths, so the code from the appropriate source
files now renders in the built documentation.