I noticed that dev-python/twitter 1.18.0 has been out for a while now. It seems like just renaming the .ebuild is enough for the version bump. I didn't run into any problems at least when manually bumping the version in my local portage tree.
commit 755fb586658a30a942cb471ef044c57fdbde84ff Author: Patrice Clement <monsieurp@gentoo.org> AuthorDate: 2019-02-12 22:04:25 +0100 Commit: Patrice Clement <monsieurp@gentoo.org> CommitDate: 2019-02-13 23:07:02 +0100 dev-python/twitter: version bump. Signed-off-by: Patrice Clement <monsieurp@gentoo.org> Package-Manager: Portage-2.3.51, Repoman-2.3.11 RepoMan-Options: --force