Windows: improve performance via buffered I/O The posixfile_nt code hits the win32 file API directly, which essentially amounts to performing a system call for every read and write. This is slow. We add a C extension that lets us use a Python file object instead, but preserve our desired POSIX-like semantics (the ability to rename or delete a file that is being accessed). If the C extension is not available (e.g. in a VPS environment without a compiler), we fall back to the posixfile_nt code.

win32: clarify comment regarding use of fdopen

win32: allow catching of both pywintypes.error and WindowsError

atomictempfile: delegate to posixfile instead of inheriting from it