First, I checked that the API does exist on Window 2000. And python does
not have any "never fail guarantee", specially in low memory conditions.

Then, some remarks:

- If Process32First or Process32Next fail for any reason, "return
win32_error(...)" is called, but the GetLastError function won't find
the cause because CloseHandle will clear it.

- It would be nice to add a comment like "if the loop ends and our pid
was not found, GetLastError() will return ERROR_NO_MORE_FILES.  This is
an error anyway, so let's raise this."

- A unit test is necessary: it could use a subprocess and compare
getpid() with the child's getppid().

- The documentation should state what happens when the parent process
exits.  Is the ppid reset to zero?  Is it possible to have another new
process with the same ppid?