Add opt-in Unicode whitespace stripping for section names (issue pytest-dev#4)

Changes:
- Add strip_section_whitespace parameter to IniConfig.parse()
  - Default: False (preserves backward compatibility)
  - When True: strips Unicode whitespace from section names
- Document Unicode whitespace handling in CHANGELOG
  - Python 3's str.strip() has handled Unicode since Python 3.0 (2008)
  - iniconfig 2.0.0+ benefits from this automatically
  - Values and key names already strip Unicode whitespace correctly
- Add tests for Unicode whitespace handling

Background:
Since iniconfig moved to Python 3 only in version 2.0.0, all strings are
Unicode by default. Python 3's str.strip() handles Unicode whitespace
characters (NO-BREAK SPACE, EN QUAD, IDEOGRAPHIC SPACE, etc.) automatically.
This addresses the core concern in issue pytest-dev#4 for values and key names.

The new strip_section_whitespace parameter provides opt-in stripping for
section names, which were not previously stripped for backward compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Consolidate __init__ to accept optional _sections and _sources parameters,
allowing parse() to simply call the constructor.

Changes:
- Add _sections and _sources optional parameters to __init__
- Compute sections and sources first, then assign once to Final attributes
- When pre-parsed data provided, use it directly (called from parse())
- Otherwise, parse the data normally (backward compatible path)
- Simplify parse() to just call constructor with pre-parsed data

This makes the code cleaner and easier to understand while maintaining
the exact same functionality and backward compatibility.

All 49 tests pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>