How can I remove python 2.7 after installing python 3.3?

You can't.

From the Ubuntu wiki / Python:

Longer term plans (e.g. 14.04)

Move Python 2 to universe, port all Python applications in main to Python 3. We will never fully get rid of Python 2.7, but since there will also never be a Python 2.8, and Python 2.7 will be nearly 4 years old by the time of the 14.04 LTS release, it is time to relegate Python 2 to universe.

This means that a lot of base packages have hard dependencies on 2.7 and it will still take a lot of time tot get things migrated. Note that Python 3 has numerous backwards incompatible changes -- it's not a regular package upgrade.

If you really want to get rid of Python 2.7, you'll have to wait for the 14.04 release, but there's no guarantee.

Came here in 2019 because I develop in Python3 by default and came to the same conclusion as OP after seeing what'd be removed after running apt purge python

Since what I really wanted was to call Python3 with just python, I ran

sudo rm /usr/bin/python
sudo ln -s /usr/bin/python3 /usr/bin/python

This way, if Python2.7 is still needed, it can be called explicitly with python2.7 while simply calling python will default to Python3 because of the symbolic link.

I don't have any bash level scripts that call python2.7 with python so this change wouldn't be disruptive - while other systems would need their scripts adjusted accordingly if they did.

The main barrier to a distribution switching the python command from python2 to python3 isn't breakage within the distribution, but instead breakage of private third party scripts developed by sysadmins and other users.

- The "python" Command On Unix-Like Systems

This answer isn't a direct response to OP, but as someone who had a similar question this is the functionality I was looking for when I was thinking of removing 2.7. Rather than delete, just prioritize which one gets to use python.