shutil: Simplify code to get closer to original
shutil: Simplify code
subprocess: Simplify code

We can now declare the type of an attribute when initializing it via
self -- good!
tempfile: Simplify code
Include platform-specifics in os stubs

Close #209.
tempfile: Simplify code
Add platform-specific stat result flags
shutil: Simplify code
subprocess: Simplify code
genericpath: Simplify code
tempfile: Refactor to be closer to original
posixpath: Minor refactoring
Declare attribute in TypeChecker