replace
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3
with
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2
[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3
e.g. installing into /usr/local/bin instead of /usr/bin.
and ensure the /usr/local/bin is before /usr/bin in PATH.
i.e.
[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin
Ensure this always is the case by adding
export PATH=/usr/local/bin:$PATH
to the end of your ~/.bashrc file. Prefixing the PATH environment variable with custom bin folder such as /usr/local/bin or /opt/<some install>/bin is generally recommended to ensure that customizations are found before the default system ones.