Update Makefile

[blag]

This PR updates the Makefile to use Python 3's venv module, and it fixes a few oversights made when adding black.

[arm4b]

Thanks! The packaging is heavily relying on the virtualenv and so the tooling consistency is the first idea that came to my mind here.

But what is our motivation behind replacing virtualenv with venv at least for the Makefile?

[blag]

I believe that the virtualenv executable is going to go away in future versions of official Python releases ¹ and the venv module will be the primary way to create virtualenvs.

Now that we're only supporting ST2 on Python 3, we should always have the venv module, and we should migrate to that before it breaks our CI process and becomes an emergency.

¹ While I'm sure somebody in the Python community will create and maintain a virtualenv executable, I don't think this is something that we should rely on long term.

[CLAassistant]

All committers have signed the CLA.

[winem]

How do we want to proceed here? I see the point and am a big fan of tooling consistency but @blag has a very good point and I don't see any real risk in switching to venv.

Is there anything else that may help to make a final decision?

[arm4b]

👍

Forgot to follow up on this.