Implement "gen.gi_frame" and "coro.cr_frame" attributes on generators and coroutines

Implement "gen.gi_frame" and "coro.cr_frame" attributes on generators and coroutines that return an inspectable (although otherwise dead) frame object.
Closes #2306.

Fix inconsistency between traced files and reported files

Fix incompatibility with C89

It fixes error 
```
src\lxml\etree.c(241210) : error C2275: 'PyObject' : illegal use of this type as an expression
```
when installing lxml 4.3.0 to pypy2 v6.0.0 on Windows.

ENH: allow @property decorator on external ctypedef classes

Change the default language level to "3str"

Enable C++ tests in appveyor, only for Py2.7/3.7 64 bit for now.

Closes #1635.