policy: mark all string literals as sysstr or bytes The policy module won't be imported early in future, which means string literals will be processed by our Python 3 loader.

debuginstall: check C extensions only if they are loadable per policy This check is useless in pure installation and I want to make it directly import C extension modules.

osutil: proxy through util (and platform) modules (API) See the previous commit for why. Marked as API change since osutil.listdir() seems widely used in third-party extensions. The win32mbcs extension is updated to wrap both util. and windows. aliases.

win32mbcs: wrap underlying pycompat.bytestr to use checkwinfilename safely win32mbcs wraps some functions, to prevent them from unintentionally treating backslash (0x5c), which is used as the second or later byte of multi bytes characters by problematic encodings, as a path component delimiter on Windows platform. This wrapping assumes that wrapped functions can safely accept unicode string arguments. Unfortunately, d1937bdcee8c broke this assumption by introducing pycompat.bytestr() into util.checkwinfilename() for py3 support. After that, wrapped checkwinfilename() always fails for non-ASCII filename at pycompat.bytestr() invocation. This patch wraps underlying pycompat.bytestr() function to use util.checkwinfilename() safely. To avoid similar regression in the future, another patch series will add smoke testing on default branch.