The thonny-ev3dev package is a plug-in which adds EV3 support for Thonny.

To correctly use the thonny-ev3dev plugin you must not use 'import ev3dev.ev3 as ev3' to import the ev3dev library, but instead you import it as:

# get ev3dev library in current context
import ev3devcontext
ev3=ev3devcontext.getEV3API()

Then depending on the context(simulator,EV3,pc) the right library is loaded.

For more info about the thonny-ev3dev plugin see: https://github.com/harcokuppens/thonny-ev3dev/wiki
For more info about Thonny: http://thonny.org