tox: 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.

Removing the option allows us to remove ignore_basepython_conflict.

Change-Id: Icd201e95f79bd07df7b8eabf8a14c869d0e32e17
Co-authored-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,pep8
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -50,15 +48,16 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/doc/requirements.txt
 setenv = PYTHONHASHSEED=0
 commands =
   rm -rf doc/build
   sphinx-build -E -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-Allowlist_externals =
-  rm
+deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html