Handle simple if statements when processing stubs #569

Typeshed's stubs contain many if statements to declare things based on the Python version and the system platform. In order to have the right type info, when analyzing stubs we should handle those checks. They're not complicated, and are usually like:

if sys.version_info >= (3, 6): ...
if sys.platform == 'win32': ...
if sys.version_info >= (3, 3) and sys.platform != 'win32': ...

See:

• https://github.com/python/typeshed/blob/master/stdlib/3/os/__init__.pyi
• https://github.com/python/typeshed/blob/master/stdlib/3/os/path.pyi

Seems like it's all sys.version_info and sys.platform.