Based on patch by Aivars Kalvāns.

Issue #20160: broken ctypes calling convention on MSVC / 64-bit Windows (large structs) Patch by mattip

Move the _loop attribute from the constructor of _SelectorTransport,
_ProactorBasePipeTransport and _UnixWritePipeTransport classes to the
constructor of the _FlowControlMixin class.

Add also an assertion to explicit that the parent class must ensure that the
loop is defined (not None)

Patch by Alex Gaynor with some modifications by me.

and above.  Patch by Tim Graham.