Python 3: Use 'filename: learnpython*.py' (no '3')

Before renaming, Python 3 filenames were 'learnpython3*.py'.

This commit removes the '3' part from the filename.

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

```
ack -H 'filename:' python.html.markdown
find . -name "python-*.markdown" -exec ack -H 'filename:' {} \;

sed -i 's/^filename: learnpython3/filename: learnpython/' \
  python.html.markdown

find . -name "python-*.markdown" -exec \
  sed -i 's/^filename: learnpython3/filename: learnpython/' {} \;
```

ja-jp/python-jp.html.markdown

@@ -11,7 +11,7 @@ contributors:
 translators:
     - ["kakakaya", "https://github.com/kakakaya"]
     - ["Ryota Kayanuma", "https://github.com/PicoSushi"]
-filename: learnpython3-jp.py
+filename: learnpython-jp.py
 lang: ja-jp
 ---