Merge serialport-platform-7176: Use twisted.python.runtime to determine
platform in twisted.internet.serialport

Author: adiroiban

Reviewer: glyph

Fixes: #7176

This just replaces ad-hoc usage of os and sys modules with a supported
method of twisted.python.runtime.platform.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43459 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb