This fixes an issue where `distutils.spawn.spawn` was not available in
the ported upload_file, which is only used when signing the data.

This also adds a test that the gpg signature command is invoked and
included in the uploaded data.

`test_upload_metadata` was written before the fixture, so this updates
the test to use the fixture.

This is a fixture to create an upload command with a patched version of
urlopen so that no HTTP queries are sent.

Rather than writing to a file in a temporary directory, we can write to
and read from an in-memory buffer, now that the encoding functionality
in write_pkg_file is fixed.

This creates a wrapper function for writing fields in the PKG-INFO file,
both to simplify the syntax and to add a point where we can inject an
encoding function in order to support Python 2.7 compatibility.

Previously this value was hard-coded to '1.0', which was inaccurate for
many packages.

Fixes #1381

This turns get_metadata_version into a method on DistributionMetadata,
populated either by inferrence (in the case of package metadata
specified in `setup`) or from the data in a specified PKG-INFO file.

To populate metadata_version from PKG-INFO, we need to monkey patch
read_pkg_file in addition to write_pkg_file.

This is the baseline, unchanged from the version in distutils.dist, to
be modified before patching.

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a separate archive for people who want to use it as a historical
reference.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