Add futures and backport to Python 2.7 test requirements

Egg include setup py

`setuptools.tests.test_build_meta` relies on the Python 3 feature
`concurrent.futures` to run, and as a result has been silently skipped
in Python 2.7. This adds the `futures` backport to the 2.7 test
requirements and turns the `pytest.importorskip` invocations in test_build_meta
into standard import statements.

Fixes GH issue #1506

This tests that `setup.py` is included by default in the distribution
with the egg_info command and when an sdist is built with
build_meta.build_sdist

Document setup.cfg minimum version for version and project_urls

Update documentation for setuptools distribution

Add missing word to sentence in python 3 docs

This adds a simple introductory guide to the modern way to distribute
`setuptools`-based projects with a link to the Python packaging
tutorial.

Rather than delete the old `ez_setup` information, it has been moved to
a separate archive for people who want to use it as a historical
reference.

Document that rpmbuild is required for bdist_rpm

Refs #1456

Deprecate the requires keyword

Custom deprecation warning classes.

- version: a960ee1c
- project_urls: df224644

These are tests to ensure that the specified deprecation warnings are
raised when the functions are called.
Co-authored-by: Junhan Huang <robin.j.huang@gmail.com>
Co-authored-by: Marton Pono <marci93@gmail.com>

`DeprecationWarning` is not visible by default in the latest versions of
CPython, so this switches the deprecation warnings in setuptools and
pkg_resources over to custom classes derived from `Warning` instead.

Fixes issue github issue #159
Co-authored-by: Junhan Huang <robin.j.huang@gmail.com>
Co-authored-by: Marton Pono <marci93@gmail.com>

remove private and redundant link

Remove ez_setup from setuptools installation docs
    
This removes ez_setup.py from the setuptools development instructions
and updates some links into EasyInstall that were skipping over the
deprecation warning.

Update recommendation.

Co-Authored-By: jaraco <jaraco@jaraco.com>

Document using setup.cfg for src-layouts