Specify python3 in installation instructions.

[shivanshu1234]

Closes #5550

[jasonaowen]

Thank you!

[stsewd]

Thanks, I wonder if can just use python -m venv venv

[jasonaowen]

Thanks, I wonder if can just use python -m venv venv

That would work (I just tested it), but we'd also need to update at least the Ubuntu required packages to include python3-venv:

$ python3 -m venv venv 
The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

    apt-get install python3-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/tmp/venv/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

[shivanshu1234]

Yes, can confirm this. Ubuntu Bug #1532231

[jasonaowen]

It looks like the python3-venv packages exists on Xenial (16.04) and later. After installing it on my 18.04 machine, I was able to run python -m venv venv successfully, and it created an environment where python was linked to the system python3.6.

[humitos]

Thanks!