Prevent multi-line descriptions for crashing putup

[abravalheri]

Purpose

Users can decide to edit setup.cfg and change the description field to be a multi-line string.
This can break updates, because the current setup.cfg template depends on the fact it is a sinlge line (the INI/CFG syntax is broken otherwise).

This issue is related to #506.

Approach

When rendering the template use just the first line of the description.
Then use ConfigUpdater to fix it, if necessary, with set_values([ ... list of lines ...]).

[coveralls]

Pull Request Test Coverage Report for Build 5711034905264128

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 97.291%

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Totals
Change from base Build 5113446040600576:	0.006%
Covered Lines:	1637
Relevant Lines:	1669

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💛 - Coveralls

[FlorianWilhelm]

Thanks. Nice solution. Makes me think if maybe we could one day completely go for ConfigUpdater as template systems for setup.cfg. I think it would save us some code and avoid workarounds like this. But I am also not sure if this change would make sense when pyproject.toml will replace it anyway.

I added some comments to the code which I think makes it easier to understand what's going on. Feel free to merge @abravalheri. And thanks a lot again :-)

[abravalheri]

Yes, that would make sense.
I suppose a purely ConfigUpdater does not make sense, because it is just easier to add the structure and the comments in the plain text, but we could avoid doing any string interpolation and just fill the option values using ConfigUpdater. It should not be super-hard...

Let's see if we have any news about PEP 621 and setuptools in the near future. If not, we might want to invest sometime on this...

Thank you very much @FlorianWilhelm for the suggestions! I think they improve a lot the PR.