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

Fix annotation_typing test case

* Add `mypy` as a tag to the `pure_py3.py` test module.
* Add type hint for MyStruct2 in pure_py3 test, because MyStruct2 is of type
  `cython.typedef` instead of `cython.StructType`, so one needs to explicitly
  provide its type.

* run mypy against test modules tagged with `mypy`

Minor C code generation/utility fixes

* `FusedType` for the PEP484's stub file

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

FIX: account for change in how exception information is stored

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

Avoid comparing arbitrary type string representations for equality and instead check for the correct instance type first.

Type inference for Pythran expressions