The xmethod Python code has few bits which are not Python 3 compatible. Note that this bug is already fixed by this https://sourceware.org/ml/gdb-cvs/2014-08/msg00038.html on trunk, and with https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=e94a94219df34ec7350518ffec3c971dd17a1fdb on branch. Filing because we need a PR for release process.
As mentioned in the previous comment, this is already fixed on trunk and the 7.8 branch.