The ebuild was renamed in the Portage tree on 09 Aug 2013:
$ cat /usr/portage/dev-python/twisted-core/ChangeLog
...
*twisted-core-13.0.0-r1 (09 Aug 2013)
09 Aug 2013; Michał Górny <mgorny@gentoo.org> +twisted-core-13.0.0-r1.ebuild:
Migrate to twisted-r1.
06 Aug 2013; Michał Górny <mgorny@gentoo.org> twisted-core-12.2.0.ebuild,
twisted-core-12.3.0.ebuild, twisted-core-13.0.0.ebuild:
Fix patch paths. Bug #479942.
*twisted-13.0.0 (08 Apr 2013)
...
# ChangeLog for dev-python/pymodbus
-# Copyright 1999-2011 Gentoo Foundation; Distributed under the GPL v2
+# Copyright 1999-2013 Gentoo Foundation; Distributed under the GPL v2
# $Header: $
+*pymodbus-9999 (16 Aug 2013)
+ 16 Aug 2013; W. Trevor King <wking@tremily.us> pymodbus-9999.ebuild :
+ Update dependency from 'twisted' to 'twisted-core'. The ebuild was
+ renamed in the Portage tree on 09 Aug 2013.
+
*pymodbus-9999 (19 Apr 2013)
19 Apr 2013; W. Trevor King <wking@tremily.us> pymodbus-9999.ebuild :
Pymodbus is now (v1.2.0) versioned in Git on GitHub :).
-# Copyright 1999-2011 Gentoo Foundation
+# Copyright 1999-2013 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $
IUSE="doc serial test"
RDEPEND="dev-python/setuptools
- dev-python/twisted[serial?]
+ dev-python/twisted-core[serial?]
"
DEPEND="${RDEPEND}
doc? ( dev-python/sphinx )