Revert "limit Twisted support to pre-14.0.0 while #718 is fixed"

This reverts commit 65f69e16095799e05de79a3f8f449f7b9edb27aa.
hold a reference to backwards compatible _contextFactory
Merge pull request #718 from dangra/twisted-14.0.0

[WIP] does it work with Twisted 14.0.0?