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/' {} \;
```

pt-br/python-pt.html.markdown

@@ -9,7 +9,7 @@ translators:
     - ["Paulo Henrique Rodrigues Pinheiro", "http://www.sysincloud.it"]
     - ["Monique Baptista", "https://github.com/bfmonique"]
 lang: pt-br
-filename: learnpython3-pt.py
+filename: learnpython-pt.py
 ---
 
 Python foi criada por Guido Van Rossum nos anos 1990. Ela é atualmente uma