Fix: support Alpine-specific version formats rejected by the Gentoo regex (fixes #59)

[shivamtiwari3]

Summary

Fixes #59.

AlpineLinuxVersion delegates validation to gentoo.is_valid(), whose regex only accepts the Gentoo version grammar. Alpine extends that grammar with several patterns that appear in real package databases (surfaced via VulnerableCode), causing InvalidVersion to be raised for valid Alpine packages.

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Root Cause

In src/univers/versions.py, AlpineLinuxVersion.is_valid() calls gentoo.is_valid(string) directly. The Gentoo regex (^(?:\d+)(?:\.\d+)*[a-zA-Z]?(?:_(p(?:re)?|beta|alpha|rc)\d*)*$) rejects these real-world Alpine patterns:

Input	Why it fails
1.9.5p2-r0 (OpenSSH portable)	Letter+digit suffix p2 after dotted version; Gentoo only allows a bare letter
5.15.3_git20200401-r0	_git is not in Gentoo's allowed suffix words
1.11-20-r0, 57-1-r2	Dash used as numeric component separator
0.12.5.-r0	Spurious dot before the -r0 revision marker
0.8.21.r2	Revision written as .r2 instead of -r2

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Solution

Override AlpineLinuxVersion.normalize() with a new _normalize_alpine_to_gentoo() helper that rewrites Alpine-specific patterns into their Gentoo-compatible equivalents before is_valid() and vercmp() see the string:

• "1.9.5p2-r0" → "1.9.5_p2-r0" (insert _ before single-letter+digit suffix)
• "5.15.3_git20200401-r0" → "5.15.3_alpha20200401-r0" (_git/_cvs/_svn snapshots treated as pre-releases)
• "1.11-20-r0" → "1.11.20-r0" (dash-as-dot separator)
• "0.12.5.-r0" → "0.12.5-r0" (strip spurious dot before revision)
• "0.8.21.r2" → "0.8.21-r2" (normalise .rN → -rN)

No changes to gentoo.py — Gentoo validation and comparison are unaffected.

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Testing

• Added test_alpine_extended_version_formats — verifies every version from issue Unsupported Alpine versions #59 parses and normalises to the expected Gentoo-compatible string.
• Added test_alpine_extended_version_comparison — verifies correct ordering (e.g. p1 < p2, git snapshot < stable release).
• All 748 existing Alpine tests still pass; the one pre-existing failure in test_enhanced_semantic_version is unrelated to this change (confirmed by reproducing on main before the patch).
• Run with: pytest tests/test_alpine.py -v

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Checklist

• Fixes the root cause (normalisation, not just validation bypass)
• New tests cover every failing version from the issue report
• All existing tests pass
• No unrelated changes
• Code style matches project conventions