Update dependencies to flexible version ranges

[tiagootto]

Summary

• Migrate all dependencies from exact pinned versions to compatible version ranges, resolving installation conflicts and known vulnerabilities in older versions
• Upgrade mistune from 0.8.4 to 3.x, adapting ConfluenceRenderer to the new API (Renderer → HTMLRenderer, renamed/reordered method signatures, escape_link → escape_url)
• Bump minimum Python version from 3.7 to 3.8 (3.7 reached EOL in June 2023)

Dependency changes

Package	Before	After
mistune	==0.8.4	>=3.0.0,<4
rich	==13.0.1	>=13.0.0,<16
rich-argparse	==1.0.0	>=1.0.0,<2
chardet	==5.1.0	>=5.1.0,<6
requests	==2.31.0	>=2.31.0,<3
PyYAML	==6.0.1	>=6.0.1,<7
gitignorefile	==1.1.2	>=1.1.2,<2
pytest-mock	==1.11.1	>=3.10.0,<4
requests-mock	==1.10.0	>=1.10.0,<2

Test plan

• All 88 existing tests pass
• Functional rendering test updated to match mistune 3.x output (minor formatting differences: HTML entity escaping of quotes, blockquote whitespace, code block trailing newlines)
• CLI smoke test confirms parsing and rendering work end-to-end