Can you try the following? Go to a clean directory, not under your virtualenv one. And get out of the virtualenv if you are already:

mkdir test-meltano
cd test-meltano
python3.6 -m venv venv
source venv/bin/activate
pip install --upgrade pip
pip install --upgrade setuptools

python --version
pip --version

pip install meltano
meltano --version

If everything looks good, continue with the Tutorial. Maybe there is an issue with virtualenv. I remember that it behaves in funny ways when you are under a directory that you have setup another virtualenv, etc