Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/legacy/docs/changelog.rst#v3180-2020-07-23

and drop basepython

Python 2 reached its EOL long time ago and we no longer expect any
user may attempt to run tox in Python 2.

Change-Id: I50a21a29bd918e82ab0590122d25e9312ccb28e1
Co-authored-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
Co-authored-by: Arnaud Morin <arnaud.morin@gmail.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,13 +1,14 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = docs
 skipsdist = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 
 [testenv:venv]
 commands = {posargs}
@@ -19,9 +20,9 @@ deps =
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
-whitelist_externals = make
+allowlist_externals =
+  make
 commands =
     sphinx-build -b latex doc/source doc/build/pdf
     make -C doc/build/pdf