preserver marker order at union

[dimbleby]

per python-poetry/poetry#5721, use of a set made the representation of markers non-deterministic.

A set is really the appropriate data structure, and this change is a bit of an uglification. But the number of elements in a marker union is very rarely going to be more than, say, 5 - so I doubt that efficiency is an issue.

An alternative that I considered was to try and impose an ordering in the __str__, but:

• markers don't order
• could do it alphabetically, but that's going to cause significantly more damage to existing unit tests than I would like to have to deal with...

I also decided that unit test wasn't very useful: a regression in the exact same way doesn't seem very likely. Can add the testcase from python-poetry/poetry#5721 if anyone thinks it's worth the trouble

[sonarcloud]

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