Installation instructions from tutorial should be updated

The current installation instructions: https://docs.pwntools.com/en/stable/install.html
The main difference is using Python3. After my last problem this time I decided to use Kali linux.
Running:
```
pip install --upgrade git+https://github.com/Gallopsled/pwntools.git
python3 -c 'from pwn import *'
```
worked. `python` without version did not work.

Please update tutorial to point to the current (up to date) installation instructions.