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`.

modules/utility/init.zsh

@@ -145,7 +145,11 @@ fi
 # Miscellaneous
 
 # Serves a directory via HTTP.
+if (($+commands[python3]); then
+  alias http-serve='python -m http.server'
+elif (($+commands[python2]); then
   alias http-serve='python -m SimpleHTTPServer'
+fi
 
 #
 # Functions