Step by step guide to install Python 3.6 and pip3 in Ubuntu
Install the necessary packages for Python and ssl: $ sudo apt-get install libreadline-gplv2-dev libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev
Download and unzip "Python-3.6.8.tar.xz" from https://www.python.org/ftp/python/ into your home directory.
Open terminal in that directory and run: $ ./configure
Build and install: $ make && sudo make install
Install packages with: $ pip3 install package_name
Disclaimer: The above commands are not tested in Ubuntu 20.04 LTS.