Modify http-server to work with python3

Since python3, the SimpleHTTPServer has been merged as its stated in the [docs](https://docs.python.org/2/library/simplehttpserver.html):

> The SimpleHTTPServer module has been merged into http.server in Python 3. The 2to3 tool will automatically adapt imports when converting your sources to Python 3.

Most systems alias/redirect "python" to the latest python installed:

```
❯❯❯ file /usr/bin/python
/usr/bin/python: symbolic link to `python3'
```

They also kept a `python{2,3}`, depending on the major version even if only one is installed. So, in a system with only `python 2.x`, with this change it should still work.

(Another alternative would be run `python2 -m SimpleHTTPServer`.)
No, I mean, notice the syntax error. The proper syntax is `if (( $+commands[python3] )); then`.