Pin towncrier to 19.9.0 (#9282)
Pin towncrier to 19.9.0 (#9282)

See https://github.com/twisted/towncrier/issues/346
Closes #8817, supersedes #9045 and #9046.
Akasurde/pytest,pytest-dev/pytest,markshao/pytest,RonnyPfannschmidt/pytest