Fix stop generating code that compiler skips

The types of variables __pyx_PickleError, __pyx_result from generated
unpickle function, and variables state, _dict from __reduce_cython__
generated method were not declared.
So warnings were raised with warn.undeclared for every single extension
type.

Now, we define the type of these variables, and no warnings are raised.

Fix warn.undeclared #2685

fix co_firstlineno for functions that have decorators

Remove support for Py2.6 and various quirks that special-cased it

Closes #2692.

Fix: Exponentiation of integer constants

The types of variables __pyx_PickleError, __pyx_result from generated
unpickle function, and variables state, _dict from __reduce_cython__
generated method were not declared.
So warnings were raised with warn.undeclared for every single extension
type.

Now, we define the type of these variables, and no warnings are raised.

Fix Memviewslice docstrings & Test

[#2634] Merge Fix 

Workaround for long internals misconfiguration on 64-bit MinGW.

Also add a compile time check for SIZEOF_VOID_P.

This fixes #2670.

See also
https://bugs.python.org/issue35037
https://bugs.python.org/issue4709

Fix reimport_from_subinterpreter test when PYTHONPATH is set