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/' {} \;
```

ro-ro/pythonlegacy-ro.html.markdown

@@ -4,7 +4,7 @@ contributors:
     - ["Louie Dinh", "http://ldinh.ca"]
 translators:
     - ["Ovidiu Ciule", "https://github.com/ociule"]
-filename: learnpython-ro.py
+filename: learnpythonlegacy-ro.py
 lang: ro-ro
 ---