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$ sudo nano /etc/apt/sources.list
# add
deb http://ftp.de.debian.org/debian testing main
$ echo 'APT::Default-Release "stable";' | sudo tee -a /etc/apt/apt.conf.d/00local
$ sudo apt-get update
$ sudo apt-get -t testing install python3.6
$ python3.6 -V
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sudo apt update
sudo apt install build-essential zlib1g-dev libncurses5-dev libgdbm-dev libnss3-dev libssl-dev libreadline-dev libffi-dev libsqlite3-dev wget libbz2-dev
wget https://www.python.org/ftp/python/3.9.1/Python-3.9.1.tgz
tar -xf Python-3.9.1.tgz
cd Python-3.9.1
./configure --enable-optimizations
make -j 12
sudo make altinstall
python3.9 --version