While attempting to diagnose something (unrelated to this issue) under python 3.6, I used the following steps to clone and build a non-root python installation:

    $ mkdir py36
    $ hg clone https://hg.python.org/cpython
    $ cd cpython
    $ hg update 3.6
    $ ./configure --prefix=/home/pi/py36
    $ make
    $ make install

Afterwards I attempted to set up a venv for further testing:

    $ /home/pi/py36/bin/python3 -m venv py36env

This succeeded but, unfortunately, activating the venv proved impossible as the "activate" script was missing from its "bin" sub-dir. 

After a bit of investigation it appears that "make install" had copied "Lib/venv/scripts/posix" but not "Lib/venv/scripts/common" into the installation, hence the templates for "activate.csh" and "activate.fish" are present, but not the "activate" template.

I had a search around for related issues but only found #22343 which explains why "activate" is no longer present (because it's moved from scripts/posix/ to scripts/common/) but I don't think is responsible for common not getting copied by "make install".

It appears adding "venv/scripts/common" to LIBSUBDIRS in Makefile.pre.in fixes the issue (as in the attached patch) but I've no idea if this covers all necessary platforms (I'd guess Windows doesn't use the Makefile if #22343 fixed things for it?).