Aqui, isso deve ajudar.
Faça o download do python 3.5
cd /usr/src
wget https://www.python.org/ftp/python/3.5.2/Python-3.5.2.tgz
sudo tar xzf Python-3.5.2.tgz
Instalar os pacotes necessários
sudo apt-get install build-essential checkinstall \
libreadline-gplv2-dev libncursesw5-dev libssl-dev \
libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev
Instale o python
cd Python-3.5.2
sudo ./configure
sudo make altinstall
Agora verifique a versão do python python -V