Python 2 has been EOL for since Jan 1, 2020, Python 3.4 has been EOL
since Mar 18, 2019 and Python 3.5 has been EOL since Sep 13, 2020.

Drop obsolete 'from __future__' imports, drop obsolete conditionals due
to Python 2 vs. 3 differences.

Virtualenv 20+ doesn't support --relocatable anymore and nothing in Pew
really depended on that.

PyPy 2.6.1's download link is not working anymore.

See https://docs.pytest.org/en/stable/yieldfixture.html.

In general, submodules and subpackages are not imported when one imports
a package, unless they are explicitly included in the package's
__init__.py.

To avoid a potential issue, explicitly import get_python_lib() from
distutils.sysconfig subpackage.

The easy_install top-level model and the easy_install console script
have been removed in setuptools 52+:
https://setuptools.readthedocs.io/en/latest/history.html#v52-0-0

Replace them with a setuptools exec that check for presence of a
setuptools dist-info, e.g. setuptools-57.4.0.dist-info.

Previoulsy, test_restore test relied on breaking the test virtualenv by
removing the site.py from the test virtualenv.

Since virtualenv 20+ doesn't inject its own site.py when creating a
Python 3 virtualenv anymore, reling on removing the site.py from the
testing virtualenv no longer works.

Deleting everything related to setuptools seems like a good way to break
the 'import setuptools' statement.

See https://docs.pytest.org/en/stable/mark.html.

Follow up on why it broke in issue:
pew-org#225.