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/' {} \;
```

uk-ua/pythonlegacy-ua.html.markdown

@@ -10,7 +10,7 @@ contributors:
     - ["habi", "http://github.com/habi"]
 translators:
     - ["Oleh Hromiak", "https://github.com/ogroleg"]
-filename: learnpython-ua.py
+filename: learnpythonlegacy-ua.py
 ---
 
 Мову Python створив Гвідо ван Россум на початку 90-х. Наразі це одна з