Avoid python command usage in install.sh

[medicalwei]

Hi,

I have a Debian installation that doesn't have python binary since Python 2 is deprecated, and that gave me the following error:

➜  ~ curl -L https://git.io/rustlings | bash
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:01 --:--:--     0
100  2923  100  2923    0     0    436      0  0:00:06  0:00:06 --:--:--   668
Let's get you set up with Rustlings!
Checking requirements...
SUCCESS: Git is installed
SUCCESS: Rust is installed
SUCCESS: Cargo is installed
SUCCESS: Rust is up to date
Cloning Rustlings at rustlings/...
bash: line 89: python: command not found
(23) Failed writing body
Checking out version ...
error: pathspec 'tags/' did not match any file(s) known to git
Installing the 'rustlings' executable...

I would prefer avoid using python command, and use python3 command when it is available.