Change 'filename:' for Python 2 (legacy)

Before renaming, all Python 2 filenames were 'learnpython-*.py'.

This commit renames them to 'learnpythonlegacy-*.py'.

To verify that the filenames were named consistently across
translations prior to this commit, and to change this:

```
find . -name "pythonlegacy*.markdown" -exec ack filename: {} \;

find . -name "pythonlegacy*.markdown" -exec \
  sed -i 's/^filename: learnpython/filename: learnpythonlegacy/' {} \;
```

es-es/pythonlegacy-es.html.markdown

@@ -6,7 +6,7 @@ translators:
     - ["Camilo Garrido", "http://www.twitter.com/hirohope"]
     - ["Fabio Souto", "http://fabiosouto.me"]
 lang: es-es
-filename: learnpython-es.py
+filename: learnpythonlegacy-es.py
 ---
 
 Python fue creado por Guido Van Rossum en el principio de los 90. Ahora es uno