Mark unused vars with `(void)x;` [should silence Coverity Scan]

Repair indexing syntax in generated Pythran code: must use "(...)" for multiple indices, but "[...]" for a single index.
Also refactor indexing code to avoid duplication.
Closes #1946.

Bug fix for deque and vector random-access iterators

travis: we should hear it if Py3.6-dev ever starts failing. (And we should eventually remove it, but it feels too early right now.)

Hard switch off IPython test if import fails. Could not find a better way to keep it from failing on appveyor.

travis: we should hear it if Py3.6-dev ever starts failing. (And we should eventually remove it, but it feels too early right now.)

Hard switch off IPython test if import fails. Could not find a better way to keep it from failing on appveyor.

Split Python compatibility adaptation code out of ModulePreamble section and into its own separate section to allow moving it around and support using CYTHON_INLINE etc. in it.

PyPy3: Register module cleanup with atexit

Travis-CI: Setup macOS builds

Updated deque to support random-access iterators and added "difference_type operator-(iterator)" for all random-access forward and reverse iterators

Trivial typo fixes

Most are non-user facing. 
Found using:
`codespell -d -q 3`

Related to #1838.