uninstall python 2.7 and install 3

: '
Ubuntu depends greatly on Python 2.7 and Python 3, so there is no good method to remove it.
   Warning: Do not even try to remove it else you will have your OS broken.

Here is my advice....
create a virtual environment for your workspaces, 
  and if you want something better, 
    Download  Anaconda or Miniconda for a better solution  (It depends on what you develop though).
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check this threads for more.
1. https://askubuntu.com/questions/1321421/how-to-remove-all-traces-of-python-from-ubuntu
2. https://askubuntu.com/questions/315015/how-can-i-remove-python-2-7-after-installing-python-3-3
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