Install python3.9 on ubuntu 18.04

python3.9 on ubuntu 18.04

## Install Python3.9 interpreter on ubuntu 18.04

[From here](http://devmartin.com/blog/2016/04/creating-a-virtual-environment-with-python3.4-on-ubuntu-16.04-xenial-xerus/), we can pretty much follow the exact same procedure.

Only this I needed to on top of that blog was the `libffi-dev` system dependency.
  

On a terminal just do the following steps:

Install dependencies:

    sudo apt install build-essential checkinstall libreadline-gplv2-dev \
	libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev \
	libbz2-dev openssl libffi-dev

Download and untar the desired version: 
	
	mkdir -p $HOME/opt
	cd $HOME/opt
	curl -O https://www.python.org/ftp/python/3.9.2/Python-3.9.2.tgz
	tar -xzf Python-3.9.2.tgz
    

Install python3.9 while keeping python3.8 as the default python3 version on ubuntu 18.04.
    
    cd Python-3.9.2/
    ./configure --enable-shared --enable-optimizations --prefix=/usr/local LDFLAGS="-Wl,--rpath=/usr/local/lib"
    sudo make altinstall


Check it was installed properly:

    >> python3.9 -V
  Python 3.9.2
    
Enjoy!