Prior to this patch, if the author or author_email were omitted from
`setup`, a malformed `PKG-INFO` would be created.

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.

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.

`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>

Some tools, such as PBR, might want to set
long_description_content_type during the parent object's
Distribution.__init__() call (during distutils setup_keywords entry
points).  However, that field is currently unconditionally overwritten
after these calls, erasing the value.

We would rather not duplicate the existing method of copying into
dist.metadata as done with project_urls.

This preserves the fields within Distribution.metadata described by
self._DISTUTIULS_UNUPPORTED_METADATA, or otherwise takes it from
arguments.

A test case that simulates setting the long description and overriding
the arguments is added.