Use python -m pip instead pip command.

It's recommand to use this way. With tox is not really necessary to do
it but it can be good to use it. See this article:

https://snarky.ca/why-you-should-use-python-m-pip/

For example python -m pip executes pip using the Python interpreter
you specified as python.

Change-Id: Icf90d76eb51c4a52c2cfda0f51f64697bd5cfdb9

tox.ini

@@ -5,7 +5,7 @@ envlist = pep8,py27,py37,tht
 
 [testenv]
 usedevelop = True
-install_command = pip install {opts} {packages}
+install_command = {envpython} -m pip install {opts} {packages}
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt