Add IniConfig.parse() with inline comment stripping and Unicode whitespace handling

[RonnyPfannschmidt]

Summary

This PR adds backward-compatible solutions for issue #55 (inline comment handling) and issue #4 (Unicode whitespace).

Changes

1. Add IniConfig.parse() classmethod (Fixes #55)

• New parse() classmethod with strip_inline_comments parameter
  • Default: True - properly strips inline comments from values
  • Opt-out: False - preserves old behavior if needed
• IniConfig() constructor maintains backward compatibility (doesn't strip comments)
• Users should migrate to IniConfig.parse() for correct comment handling

Example:

# Recommended: strips inline comments
config = IniConfig.parse("setup.cfg")
# "name = value # comment" → value is "value"

# Backward compatible: preserves old behavior  
config = IniConfig("setup.cfg")
# "name = value # comment" → value is "value # comment"

2. Add strip_section_whitespace parameter (Addresses #4)

• Opt-in parameter for IniConfig.parse() to strip Unicode whitespace from section names
• Default: False - preserves existing behavior
• When True: strips Unicode whitespace (U+00A0, U+2000, U+3000, etc.) from section names
• Documents that Python 3's str.strip() has handled Unicode whitespace since Python 3.0 (2008)
• Since iniconfig 2.0.0 (Python 3 only), values and key names already benefit from Unicode-aware stripping

Example:

# Opt-in to Unicode whitespace stripping for section names
config = IniConfig.parse("setup.cfg", strip_section_whitespace=True)
# "[section\u00a0]" → section name is "section" (NO-BREAK SPACE stripped)

3. Code Refactoring

• Consolidated __init__ to accept optional _sections and _sources parameters
• Simplified parse() to call constructor with pre-parsed data
• Removed complex __new__ logic for cleaner, more maintainable code
• Added parse_ini_data() helper function to eliminate code duplication

Testing

• ✅ All 49 tests pass (42 existing + 7 new)
• ✅ 100% backward compatible
• ✅ All pre-commit hooks pass (ruff, mypy)

Documentation

• Updated CHANGELOG with detailed notes for v2.3.0
• Documented Python 3's Unicode whitespace handling history
• Added comprehensive docstrings with examples

Closes

Fixes #55
Addresses #4 (opt-in solution with full Unicode whitespace documentation)

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