I replied to @mrits because @mrits was presenting virtualenv as replacement for using pyinstaller to manage the installation of non-OS-managed Python versions (which @cowsandmilk was talking about).
I'm unsure how what I said:
> If you want another Python, then you need to install it yourself (somehow) and then point virtualenv at it.
Differs from what you are saying:
> 'virtualenv -p /usr/bin/python3.5 venv'
Then create a folder for your project, 'virtualenv -p /usr/bin/python3.5 venv' to create the venv in that directory. Activate/deactivate the environment as you wish when you want go between the system Python environment and the one in your folder.