linelog: fix import statements to pass the import checker on py3

Differential Revision: https://phab.mercurial-scm.org/D4050

mercurial/linelog.py

@@ -23,12 +23,12 b' from __future__ import absolute_import, '
 import abc
 import struct
 
-from mercurial import (
-    pycompat,
-)
 from .thirdparty import (
     attr,
 )
+from . import (
+    pycompat,
+)
 
 _llentry = struct.Struct('>II')