Tests: use package metadata as source of README test data.

[jayaddison]

This works for me locally on Py3.11; I'm not 100% certain whether this is entirely backwards compatible through Py3.8.

May resolve #1160.

[jayaddison]

@rmdluo @jknndy the README.rst file isn't included in the built recipe_scrapers Python library package -- however, the same metadata is available in the package description, and the test_includes unit test (the one that checks for all of the scraper hyperlinks) does work using that text instead.

What do you think about this as a potential change to read the content from the package metadata?

[jayaddison]

This should be a reversible decision, so I'm going to move ahead with it in order to resolve #1160. If we later discover that we have to backtrack/revert, that's OK.