versions: preserve any local label when bumping

[abn]

Split out of #379

[sonarqubecloud]

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[dimbleby]

What is this fixing?

I don't think it's obvious what the "right" behaviour is, or even that there is a single right behaviour

• eg users of poetry version might - or might not - want this updated behaviour
• there are internal places that call next_minor() and next_major() as part of version comparison, does this change help or hurt them?

[radoering]

Further, according to PEP 440 local version identifiers are not permitted in inclusive and exclusive ordered comparisons. Thus, the tested constraints are invalid according to PEP 440. If this behavior is required for some reason, we should at least add a comment explaining why it's required.