mercurial/tags.py

--- a/mercurial/tags.py	Thu Jun 20 14:45:52 2019 -0700
+++ b/mercurial/tags.py	Thu Jun 20 14:33:42 2019 -0400
@@ -13,6 +13,7 @@
 from __future__ import absolute_import
 
 import errno
+import io
 
 from .node import (
     bin,
@@ -562,7 +563,7 @@
             " branch name\n") % name)
 
     def writetags(fp, names, munge, prevtags):
-        fp.seek(0, 2)
+        fp.seek(0, io.SEEK_END)
         if prevtags and not prevtags.endswith('\n'):
             fp.write('\n')
         for name in names: