use tox-wheel 0.6 isolated_build flag

tox-wheel used to use its own flag to disable legacy builds, it
now uses the tox native config option:

ionelmc/tox-wheel@97a9e89

Author: graingert

pyproject.toml

@@ -80,6 +80,7 @@ legacy_tox_ini = """
 envlist =
     py{36,37,38},lint
 minversion=3.20.1
+isolated_build=true
 requires=
     virtualenv >= 20.1.0
     tox-wheel >= 0.6.0
@@ -89,7 +90,6 @@ requires=
 extras = test
 commands = pytest {posargs}
 wheel = True
-wheel_pep517 = True
 wheel_build_env = build
 
 [testenv:build]