[NLPL Task Force (A)] adding virtualenv to python-env/2019.3

[Stephan Oepen]

dear colleagues,

as part of our NeIC-funded NLPL project, we are about to install a
collection of discipline-specific software modules on Puhti.  many of
these are Python add-ons, and in the past we have had good success
creating modular 'stacks' of virtualenv instances.  i realize that
many folks consider virtualenv superseded by venv, but i would
nevertheless like to ask whether you could add virtualenv to the
python-env/2019.3 module on Puhti?  on other NLPL systems, we are
still supporting older Python versions (which lack venv), and in
general our largely automated software installation scripts currently
are built around virtualenv.

with thanks in advance, oe (for the NLPL infrastructure task force)