Fix "residual" invalid dynamic behaviour of optional-dependencies

[abravalheri]

Summary of changes

This is reminiscent of #3306.
It is another situation where PEP 621 does not allow mixing dynamic and static configurations.

Without explicitly listing optional-dependencies in dynamic users cannot rely on setup.py to specify extras_require -- in turn, when optional-dependencies is present in dynamic, then it cannot be listed as an static configuration.

Therefore, the only valid situation according to PEP 621, is that static optional-dependencies are honoured exactly as provided, without modifications (this means ignoring any extras_require previously set)..

Closes #3300

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
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[abravalheri]

Maybe this cannot be done directly this way...
I need to consider #3204.

[abravalheri]

Maybe this cannot be done directly this way... I need to consider #3204.

OK, so this should no longer be a problem since #3903.
This is because converting requirements with env markers to extras no longer is done in _finalize_requires but rather when writing core metadata and it also does not modify the original value of the distribution attribute.