Minor changes to try/finally code generation [should silence Coverity]

Minor C code generation/utility fixes

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

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.

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.

PyPy3: Register module cleanup with atexit

Travis-CI: Setup macOS builds

Use complete IPython dependency path in test runner to work around current test failures under appveyor.

Closes #1930.

Simplify preprocessor code that avoids the "fallthrough" C compiler warnings, assuming that "__has_attribute()" is widely supported these days. At least GCC 5.x and clang seem to have it.
Closes #1930.

Closes #1930.

Fix the first argument special method signatures of "__eq__", "__lt__" and the other richcmp methods to be of "self" type, in accordance with to the CPython specs.
See https://docs.python.org/3/reference/datamodel.html#emulating-container-types
Closes #1935.