I meant that python3-pep8 should be added to --skip-keys, like this: rosdep install --from-paths src --ignore-src --rosdistro dashing -y --skip-keys "console_bridge fastcdr fastrtps libopensplice67 libopensplice69 rti-connext-dds-5.3.1 urdfdom_headers python3-pep8"
The pip package will satisfy the requirement, but rosdep isn't configured to look for it, which is why the key needs to be skipped. So you should be good here.
The --rosdistro argument to rosdep is really only useful on Ubuntu right now, and I don't think it does what you want it to do even when it does work. You need to update the sources in your src directory to Foxy. The ament_pep8 package doesn't exist in Foxy, so the sources are definitely older than Foxy.
Whichever Python version is used when you invoke the "python3" executable is the one that ROS 2 will use, I believe.