* fix

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

* suppress proc related failures on pypy

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

This shouls avoid SSL failures with HTTP requests from pip, requests
or other libraries.

Also sorts existing values in order to make it easier to update the
list in the future.

Fixes: #1437

When the original interpreter [0] tox is called with is actually a pyvenv
it will set the `__PYVENV_LAUNCHER__` environment variable to the path of
pyvenv interpreter.

If tox configuration specifies `requires` and the requirements are not
satifsfied by [0], tox will create an intermediate virtual environment [1].

After setting up [1] tox will delegate the original call by spawning
a new [1] interpreter with the identical identical list of arguments.

It will also pass the value of `__PYVENV_LAUNCHER__` if present. This in turn
causes `sys.executable` to resolve to [0] instead of [1] therefore ignoring all
modifications including installed requirements.

The fix is to make sure that when [0] spawns [1], `__PYVENV_LAUNCHER__` is
removed from the environment.

`subprocess` reports a negative exit code for signals.

Still handle "128 + X" for processes indicating that they received a
signal themselves.

...but keep the TOX_PARALLEL_ENV variable for others to consume.

The problem with using environment variables to let tox know it's being run
as a parallel worker is the following:

Either you don't pass the environment variable, but the test frameworks and
test suites are unaware that tox is running in parallel.
This makes it hard for cases like pytest-django:
https://github.com/tox-dev/tox/pull/1139

Or you pass the environment variable, but it makes tox invoked within tox
think it is a parallel worker even when it isn't.
This makes it hard to test various tox plugins that invoke tox in their
integration test suite:
Fixes https://github.com/tox-dev/tox/issues/1444

By introducing two variables we can use one ("private", _TOX_PARALLEL_ENV) to
control the behavior of tox (and don't pass it) and another ("public",
TOX_PARALLEL_ENV) to let the tests and test frameworks know tox is running
in parallel (and pass it by default).

The integration test is a tad complicated invoking subprocess.Popen instead
of subprocess.run, to support Python 2.7 and 3.4.

* Upgrade minimal virtualenv version to 16.0.0

Fixes #1425

* Add a change note about the virtualenv bump