Upgrade to python 3.6 wants to delete python3-dev & python3-venv

[Jim]

This happened on a machine running Mint 18.1, I haven't had a chance to
check if it happens on one running Ubuntu.

I installed python 3.6 to use in a virtual environment. Now an update
wants to delete python3-dev and python3-venv. Checking to see if anyone
has done the update and whether or not it has caused any problems.

Thanks, Jim