Add CYTHON_NO_PYINIT_EXPORT macro. (#1944)

CPython change how the exception information is stored internally in
3.7a3.

This simply adds fences based on python version around how to access
the objects.

Xref:

https://github.com/python/cpython/commit/ae3087c6382011c47db82fea4d05f8bbf514265d
https://bugs.python.org/issue25612
https://github.com/python/cpython/pull/1773

FIX: account for change in how exception information is stored

CPython change how the exception information is stored internally in
3.7a3.

This simply adds fences based on python version around how to access
the objects.

Xref:

https://github.com/python/cpython/commit/ae3087c6382011c47db82fea4d05f8bbf514265d
https://bugs.python.org/issue25612
https://github.com/python/cpython/pull/1773

Avoid comparing arbitrary type string representations for equality and instead check for the correct instance type first.

Type inference for Pythran expressions

Make it possible to pass the type together with the index into _index_type_code, as this is needed during type interference

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.

When building the generated code in an embedded interpreter the module
init function will no longer be exported if CYTHON_NO_PYINIT_EXPORT is defined.

See https://github.com/cython/cython/issues/1944 for details.