…e issue analysis

This commit improves the stream2py codebase with bug fixes, new features,
enhanced test coverage, and comprehensive issue analysis.

## Bug Fixes
- fix: Python 3.11+ compatibility in test_util.py - handle TypeError for context manager protocol
- fix: add missing open_instance attribute to QuickSourceReader
- fix: SimpleSourceReader now returns None instead of raising StopIteration
- docs: fix syntax error in __init__.py docstring (missing colon)

## Features
- feat: add closed property to SourceReader (#6)
  - Added closed property similar to io.IOBase.closed
  - Updated __enter__ and __exit__ to manage _closed flag
  - Updated QuickSourceReader to properly set closed state
  - Updated docstring with usage example

## Tests
- test: add comprehensive tests for BufferReader blocking parameter
  - Tests for blocking=True/False behavior
  - Tests for blocking read when buffer stops
- test: add comprehensive tests for QuickSourceReader
  - Tests for basic functionality, context manager, info, key, iteration
  - Tests for custom is_valid_data filtering

## Documentation
- docs: create comprehensive ISSUE_ANALYSIS.md
  - Analyzed all 17 open issues
  - Categorized by status, effort, and dependencies
  - Identified already-resolved issues (#20, partial #8)
  - Provided resolution recommendations for each issue

## Test Results
- Before: 11 tests (1 failing)
- After: 19 tests (all passing)
- 8 new tests added
- All existing tests fixed and passing

## Issues Addressed
- Resolves #6 (closed property for SourceReader)
- Documents that #20 is already resolved (blocking parameter exists)
- Documents that #8 is partially resolved (blocking exists, timeout could be added)