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).

[WIP] Add TSS (Thread Specific Storage) API in CPython 3.7+

Closes #1949.

Closes #1968.

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

Fix annotation_typing test case

Minor C code generation/utility fixes

Move CYTHON_NO_PYINIT_EXPORT utility code into external file to avoid lengthy chain of plain text code generation.

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