Add 'except' values for various function types

[travis] update gcc

Remove direct Unicode hash usage for dict lookups as it proved to be slower after all. Optimising a generic item lookup for dict is faster, though.

See #1979.

Split up the module init function into separate C functions per setup step as it can get fairly lengthy for complex modules, e.g. with many extension types.
This hopefully reduces the function complexity that the C compiler must handle.

Closes #1955.

Add 'except NULL' to PyWeakref_GetObject.

Update implementation of PEP 3118 getbuffer special method

* Rework check for `view==NULL` required for legacy Python 3 releases.
* Handle redefined `struct Py_buffer` in user code (e.g. mpi4py).