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/' {} \;
```

python.html.markdown

@@ -9,7 +9,7 @@ contributors:
     - ["Rommel Martinez", "https://ebzzry.io"]
     - ["Roberto Fernandez Diaz", "https://github.com/robertofd1995"]
     - ["caminsha", "https://github.com/caminsha"]
-filename: learnpython3.py
+filename: learnpython.py
 ---
 
 Python was created by Guido van Rossum in the early 90s. It is now one of the most popular