Python 3 dependency bloat

The closure of the Python 3 package is much bigger than Python 2:

```
$ du -sch $(nix-store -qR $(nix-build -A python27))
110M    total

$ du -sch $(nix-store -qR $(nix-build -A python34))
269M    total
```

This is primarily due to the dependency on TCL/TK, which pulls in libX11 and other X11 libraries.
