In the documentation for venv (https://docs.python.org/3.5/library/venv.html) it gives examples for activating the venv under bash/zsh.

For a long time under both Debian and Ubuntu the default shell has been Dash (https://wiki.ubuntu.com/DashAsBinSh)

Dash doesn't undertsand the command 'source' so instead the venv must be activated using the . command (as is done for Fish, but using the bash activate file instead)