[Twisted-Python] pyserial on Python3

[Adi Roiban]

Hi,

We have this ticket which ports twisted.internet.serialport on Python
3 - https://twistedmatrix.com/trac/ticket/8099

I saw that the Python 3 builders don't have pyserial installed.

Was this on purpose or just forgetting to also install
.[all_non_platform] on the python 3 builders?

Thanks!