always use UtilityCode.load_cached() for reading dependencies of helpers as it's highly likely that they will be reused by others as well
fix transitive loading of parametrised utility code
fix some pyregr tests by copying required (data) files

--HG--
extra : amend_source : b920a1b9cf4249bb95a7f1a310222080c26dc326
improve some Sphinx markup
first start at a string tutorial section on accepting strings from Python code
fix sphinx reference
sphinx formatting fix
clarify memory view section in string processing docs
copy some more test files in pyregr tests
fix in-place assignments of arbitrary objects to variables with inferred Python builtin/extension types
use symlinks instead of copying test files if possible
remove copied HTML test data files after running test
CF: Fix try/except, try/finally bug
CF: reference doesn't mean it's not null