Fixes Github issue #1886.

Closes #1879.

Disable PEP-489 support by default (*sigh*).  It actually requires more than Cython currently implements: sub-interpreter support and reloading support, i.e. per-interpreter module globals/types/state.
Closes #1894.

Make test runner shards deterministic with a non-forking multiprocessing pool (i.e. on Windows) that may use different hash seeds.

Rewrite tp_dealloc() of fused types functions to correctly disable GC before starting to clean up references.

Fix Py3 list comprehensions in (C-)class bodies: loop variable was declared in the object struct instead of the scope that declared the class (usually the module init function).

Prevent Cython types in annotations from ending up plainly in the __annotations__ dict. Convert them into strings instead.

Add test for ticket #1888: mismatching argument names between .pxd and .pyx files are (not great but) actually ok, but the callers must always see the "public" pxd names, and the implementation must see and use the names given by the implementation.

Closes #1887.

Avoid depending on CPython micro level versions at compile time. Compiling against 3.5.2 does not guarantee that we never get imported on 3.5.1.
Closes #1880.