Point pip command to python 3.8 from 3.6 on ubuntu 18.04

• python3.6 on ubuntu18.04 is necessary so you CANNOT remove that version(it includes system libraries)
• prerequires: installed python3.8.
• so you've 2 python 3.x(3.6, 3.8)

1. check current state
   $pip -V
   (out)pip 20.0.2 from /home/amaruak00/.local/lib/python3.6/site-packages/pip (python 3.6)

2. install venv, dev
   $sudo apt-get install python3.8-venv python3.8-dev

3. update 3.8 pip
   $sudo /usr/bin/python3.8 -m pip install --upgrade pip
   (upgrading)

4. check
   $pip -V
   (out)pip 20.0.2 from /home/amaruak00/.local/lib/python3.8/site-packages/pip (python 3.8)

5. setting python3.8 as python3
   $sudo update-alternatives --config python3
   $sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8
   #check python3.8 as primarly

   • If some libraries are seems crash, uninstall & reinstall
   • (ex)pip uninstall Pillow / pip install Pillow