next_version("prerelease") returns a lower version when switching to an earlier prerelease token #497

Which version of Python is the problem with?

3.9 (not version specific)

What semver version are you using?

master (84561d5)

What OS are you using?

macOS

Situation

Since #472 (fix for #339), next_version("prerelease", prerelease_token=...) resets to <token>.1 whenever the token differs from the current one. When the new token sorts before the current one, the result is a lower version, which contradicts the docstring ("Determines next version, preserving natural order").

How to reproduce

>>> from semver import Version
>>> v = Version.parse("1.2.3-rc.1")
>>> n = v.next_version("prerelease", prerelease_token="alpha")
>>> n
Version(major=1, minor=2, patch=3, prerelease='alpha.1', build=None)
>>> n > v
False

Same for 1.2.3-beta.3 -> alpha (1.2.3-alpha.1) and 1.2.3-rc.1 -> dev (1.2.3-dev.1). dev -> rc works only because rc sorts after dev.

Expected behavior

next_version always returns a version greater than self. Two options I can see when <token>.1 would not be greater:

1. Bump the patch and start the new token there: 1.2.3-rc.1 -> 1.2.4-alpha.1 (mirrors what next_version already does for a version without a prerelease).
2. Raise ValueError, since going from rc to alpha within the same release is probably a mistake.

Which do you prefer? Happy to send a PR with tests and a changelog entry.