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/' {} \;
```

pt-br/pythonlegacy-pt.html.markdown

@@ -5,7 +5,7 @@ contributors:
 translators:
     - ["Vilson Vieira", "http://automata.cc"]
 lang: pt-br
-filename: learnpython-pt.py
+filename: learnpythonlegacy-pt.py
 ---
 
 Python foi criado por Guido Van Rossum no começo dos anos 90. Atualmente é uma