osutil.c: include Python.h before the other headers
This is recommended by the Python documentation, since Python.h defines
some constants.
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'