Update the minversion parameter.

Update the minversion parameter to use the python -m pip to install
python packages:

https://tox.readthedocs.io/en/latest/changelog.html#id185

It's recommend to use this. Remove the useless install_command
parameter.

Change-Id: Iebc9d821a7f732b44c61c69337dc9f305aa70658

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.2.0
 envlist = py37,pypy,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -7,13 +7,12 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
-setenv = VIRTUAL_ENV={envdir}
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals = find
+setenv = VIRTUAL_ENV={envdir}
 commands =
     find . -type f -name "*.py[c|o]" -delete
     find . -type d -name "__pycache__" -delete