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a separate archive for people who want to use it as a historical
reference.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

For runtime dependencies, install_requires should be used. For build
dependencies, a PEP 518-compliant `pyproject.toml` should be used.

Other dependencies can use extra requirements.

Document building Cython projects instead of Pyrex

Exclude .pyc in sdist output

Added minumum column in metadata table and version info for long_description_content_type

This also documents that the long_description_content_type keyword was
added in version 38.6.0.
Co-authored-by: Gene Matusovsky <gene_matusovsky@s5a.com>

This should stop the PyPI release stage if a `.pyc` exists in the
generated SOURCES.txt.

Mark Py 2/3-only tests as skip instead of xfail