Add support to declare multiple README files

[wagnerluis1982]

Resolves: python-poetry/poetry#873

• Added tests for changed code.
• Updated documentation for changed code (Add docs and schema for multiple readme files support poetry#4743).

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This code reuses the "readme" entry in a way to allow the user to declare in the old way or the new way. The two following declarations become valid:

Single file

readme = "README.rst"

Multiple files

readme = [
    "README.rst",
    "HISTORY.rst"
]

If the user declares files in different formats, the strict validation will issue.

[sinoroc]

Where should I add documentation? Should I open another MR in the main project for that?

I think so, yes.

[wagnerluis1982]

I rebased my branch to latest master and solved conflicts. Also added documentation to python-poetry/poetry#4743

[wagnerluis1982]

Thanks for the awesome suggestions.

I added my fixes to the code: please note that I rebased the branch to reduce the number of commits.

Comments about my fixes:

• I chose isinstance(config["readme"], list) as the TOML parser only delivers list-like objects, no need to check for tuple.
• About using a Sequence, please check my separate comment
• Pluralized Package.readme (became readmes) and readme_type => description_type
• Added a test for validation fail when readme types mismatch
• Refactoring according to the comments
• Fixed a few glitches I found on the way