Dynamic readme value generation

[dataflake]

This PR improves generating the readme attribute for a package:

• if there's just a readme file, put readme into the [project] section with the readme file name. This is what's happened so far.
• if there's a readme and a change log file, put them into the [tool.setuptools.dynamic] section so both get used. This is similar to what most packages did with the long_description setting in setup.py.

The PR does not attempt to parse previously used file names out of the long_description value because the format is undefined, every package did it slightly differently. It just assumes that maintainers want to concatenate the readme and the change log as content for the PyPI project page.

If the package did something entirely different it is up to the script user and/or the PR reviewers to catch the difference and correct that manually.

[coveralls]

Pull Request Test Coverage Report for Build 19765675198

Details

• 0 of 20 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 14.867%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/zope/meta/setup_to_pyproject.py	0	20	0.0%

Totals
Change from base Build 19761874204:	-0.2%
Covered Lines:	240
Relevant Lines:	1218

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💛 - Coveralls

[dataflake]

I'm going to merge this myself because it is tested and works just as intended (see https://pypi.org/project/ExtensionClass/6.2.1a2/), I have also manually applied the same changes to all converted repositories that currently use just readme = 'README.rst'.