"envlist" default with "isolated_build = true" broken

[Unrud]

If the environment list is not specified it defaults to python. The following tox.ini works fine:

[testenv]
commands = python --version

On the other hand, the following tox.ini doesn't work:

[tox]
isolated_build = true

[testenv]
commands = python --version

The default python environment is added, if all_envs is empty:

tox/src/tox/config/__init__.py

Lines 1499 to 1500 in 82c26ed

	if not all_envs:
	all_envs["python"] = None

But with isolated_build = true the environment .package (or whatever the value of the option isolated_build_env is) gets added to env_list a few lines earlier (making it no longer empty):

tox/src/tox/config/__init__.py

Lines 1496 to 1498 in 82c26ed

	for section in self._cfg:
	if section.name.startswith(testenvprefix):
	all_envs[section.name[len(testenvprefix) :]] = None

The section testenv:.package is added here:

tox/src/tox/config/__init__.py

Lines 1409 to 1413 in 82c26ed

	if config.isolated_build is True:
	name = config.isolated_build_env
	section_name = "testenv:{}".format(name)
	if section_name not in self._cfg.sections:
	self._cfg.sections[section_name] = {}