Closes #1972.

Multiple inheritance

Previously this was only possible through Python (requring the GIL),
sometimes implicitly (e.g. for string literals).

This fixes #1986.

We should be doing this for pure Python defined classe as well,
but don't have a way to inject it.

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.