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/' {} \;
```

zh-tw/pythonlegacy-tw.html.markdown

@@ -7,7 +7,7 @@ contributors:
     - ["evuez", "http://github.com/evuez"]
 translators:
     - ["Michael Yeh", "https://hinet60613.github.io/"]
-filename: learnpython-tw.py
+filename: learnpythonlegacy-tw.py
 lang: zh-tw
 ---