replace pytest-xdist with pytest-forked

Quote from <https://github.com/pytest-dev/pytest-xdist/blob/v3.3.1/example/boxed.txt>:

>  Since 1.19.0, the actual implementation of the ``--boxed`` option has been moved to a
>  separate plugin, `pytest-forked <https://github.com/pytest-dev/pytest-forked>`_
>  which can be installed independently. The ``--boxed`` command-line option is deprecated
>  and will be removed in pytest-xdist 3.0.0; use ``--forked`` from pytest-forked instead.

Author: Changaco

tox.ini

@@ -5,10 +5,10 @@ skipsdist=True
 [testenv]
 passenv = LIBARCHIVE
 commands=
-    python -m pytest -Wd -vv --boxed --cov libarchive --cov-report term-missing {toxinidir}/tests {posargs}
+    python -m pytest -Wd -vv --forked --cov libarchive --cov-report term-missing {toxinidir}/tests {posargs}
     flake8 {toxinidir}
 deps=
     flake8
     pytest
     pytest-cov
-    pytest-xdist
+    pytest-forked