Update another reference to Ubuntu and Python

Luckily the base images should no longer include Python 2 out of the
box, but the command `python` still typically refers to Python 2.

python/README.md

@@ -23,7 +23,7 @@ a simple language.
 
 ### Python 3
 
-Make sure to use `pip3` and `python3` when writing your code, because our autograder base image does include Python 2 as well, which are currently the defaults for `pip` and `python`. When installing Python 3, use the apt packages `python3` and `python3-pip`. If you need a more recent version than what is packaged by Ubuntu 18.04, you can try using a PPA or installing from source.
+Make sure to use `pip3` and `python3` when writing your code, because in Ubuntu Python 2 is currently the default for `pip` and `python`. When installing Python 3, use the apt packages `python3` and `python3-pip`. If you need a more recent version than what is packaged by Ubuntu, you can try using a PPA or installing from source.
 
 ## Example Test