Update readonly_writable.py for ST3

Running 2to3.py (the python conversion script) does the attached changes and makes your plugin compatible with Sublime Text 3.
Merge branch 'patch-1' of https://github.com/NicholasBuse/sublimetext-2-readonly-writable into NicholasBuse-patch-1

Conflicts:
	readonly_writable.py
Merge branch 'NicholasBuse-patch-1'