CI: run unit tests on Twisted trunk / clear separation in pinned vs latest

[oberstet]

The following tests would be good to run in CI:

https://github.com/crossbario/crossbar/blob/master/.github/workflows/main.yml => test-amd64

We could probably restructure this to use 2 different requirements:

• https://github.com/crossbario/crossbar/blob/master/requirements-pinned.txt vs
• https://github.com/crossbario/crossbar/blob/master/requirements-latest.txt

if we just added git+https://github.com/twisted/twisted to requirements-latest.txt - that is, not only run latest master for our own stuff, but also for Twisted.

[oberstet]

won't fix. we test twisted trunk vs release etc already in autobahn - that should be good enough