I've installed pytest 2.3.4 under Debian Linux.  By default it runs under Python 2.7, but sometimes I'd like to run it under Python 3.x, which is also installed.  I can't seem to find any instructions on how to do that.
The PyPI Trove classifiers show Python :: 3 so presumably it must be possible.  Aside from py.test somedir/sometest.py, I can use python -m pytest ..., or even python2.7 -m pytest ..., but if I try python3 -m pytest ... I get
/usr/bin/python3: No module named pytest
python3 doesn't have the module py.test installed.  If you can, install the python3-pytest package.
If you can't do that try this:

Install virtualenv
Create a virtualenv for python3

virtualenv --python=python3 env_name

Activate the virtualenv

source ./env_name/bin/activate

Install py.test

pip install py.test

Now using this virtualenv try to run your tests
Pytest and Python 3