logging: use warning instead warn

The warn method is deprecated. We should use the documented warning instead.

Quoting from the python's official doc:
"""
Note: There is an obsolete method warn which is functionally identical to warning.
      As warn is deprecated, please do not use it - use warning instead.
"""

(From OE-Core rev: cc771aa4b74f222f1bea38b0b50196b2fbc97ab4)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/engine.py

@@ -506,7 +506,7 @@ class Disk:
                     sparse_copy(partfname, target, seek=part['start'] * self._lsector_size)
                     os.unlink(partfname)
                 elif part['type'] != 'f':
-                    logger.warn("skipping partition {}: unsupported fstype {}".format(pnum, fstype))
+                    logger.warning("skipping partition {}: unsupported fstype {}".format(pnum, fstype))
 
 def wic_ls(args, native_sysroot):
     """List contents of partitioned image or vfat partition."""