Fix 403 error in load_features() by adding proper User-Agent header

[amansrivastava17]

Summary

• Fixed 403 error occurring in load_features() method on self-hosted GrowthBook instances
• Added proper User-Agent header to HTTP requests made by the Python SDK
• Headers now include GrowthBook-Python-SDK/{version} format

Problem

The Python SDK was failing with 403 errors when calling load_features() because urllib3.PoolManager() sends minimal headers by default. Some servers (particularly self-hosted GrowthBook instances with WAF/nginx protection) block requests without proper User-Agent headers.

Solution

Modified the _get() method in FeatureRepository class to include a proper User-Agent header for all HTTP requests.

Testing

• [x] MyPy type checking: No new type errors introduced
• [x] Test suite: All existing tests pass, no functionality broken
• [x] Flake8 linting: No new linting violations
• [x] Manual testing: Confirmed load_features() now works on self-hosted instances
• [x] Feature evaluation and experiment tracking functioning properly

Impact

• Resolves load_features() failures on self-hosted instances
• Maintains backward compatibility
• Improves SDK reliability across different server configurations
• No breaking changes to existing functionality