osutil.c: include Python.h before the other headers
This is recommended by the Python documentation, since Python.h defines
some constants.
% create heads
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
% merge heads
HG_MY_ISLINK 1
HG_OTHER_ISLINK 0
HG_BASE_ISLINK 0
merging l
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)