Add proper twisted extras requirements

[oberstet]

Currently, we have a dependency on plain twisted[], while we in fact use stuff from Twisted which is in extras - and that only worked by accident because we have other explicit or indirect requirements that happen to be the same as Twisted extras.

We should fix that properly, eg this one #1555 actually is a consequence of our incorrect dependencies.

The Twisted extras we do want are:

• all platforms: twisted[tls,conch,http2]
• additionally on osx: twisted[osx_platform]
• additionally on windows: twisted[windows_platform]

ref: https://github.com/twisted/twisted/blob/c25dcb9d4a2388710c7b2e4a2bd7242ec6747b50/src/twisted/python/_setup.py#L90

[oberstet]

fixed via #1570