Update min version of tox

This patch bumps the minimum version of tox to 3.18.0 [1].
It removes configuration of "install_command" option in tox, as it
turns out that this is the default value provided by tox [2].

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23
[2] https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: Ia5c5eb499bca5b1319098d86a2f2035f2d5d5efe

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py38,py36,pep8,docs
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -11,7 +11,6 @@ setenv = VIRTUAL_ENV={envdir}
          LANGUAGE=en_US:en
          LC_ALL=C
 usedevelop = True
-install_command = pip install {opts} {packages}
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt