Remove basepython def from tox.ini

It seems that with tox 4.2.6 the missing interpreter error was fixed but
the generative testenv feature is broken and the
[testenv:functional{,-py38,-py39,-py310}] format is leads to missing
interpreter error. It is due to a conflict between basepython = python3
and the version fragment in the generative target suppressed by
ignore_basepython_conflict = true.

This patch removes basepython = python3 assuming that developers already
switched for python3 in their environment as python2.7 is EOL.

Also we took the opportunity to add the global constraints via the
install_command instead of deps as deps is not used during the
installation of the editable package.

Change-Id: I258a7c13434b29402804181dea275b42d5539df0

tox.ini

@@ -1,20 +1,16 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,functional,pep8
-# Automatic envs (pyXX) will only use the python version appropriate to that
-# env and ignore basepython inherited from [testenv] if we set
-# ignore_basepython_conflict.
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 allowlist_externals =
   bash
   find
   rm
   env
   make
+install_command = python -I -m pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   LANGUAGE=en_US
@@ -26,8 +22,6 @@ setenv =
 # TODO(stephenfin): Remove once we bump our upper-constraint to SQLAlchemy 2.0
   SQLALCHEMY_WARN_20=1
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-  -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
 extras =
   zvm