This fixes #1788.

This fixes #1790.

Add st_birthtime to struct stat

This is available on OS X, FreeBSD and NetBSD. On Linux, the definition
itself will not result in any errors. However, code that does attempt to
st_birthtime under Linux will now fail at C compile time rather than
Cython compile time.

implement PEP 489 multi-phase module initialisation in Py3.5+

PEP 525: asynchronous generators

Update embedsignature directive to emit function annotations

Do not evaluate DEF statements inside of IF blocks with false condiions

Implement len(memoryview) and allow its usage in nogil sections.

Closes #1791.

Closes #1806.

Closes #1733

adapt finalisation code to what CPython 3.6/7 does, fix error display if not-awaited warning turns into an error