chore: bump version to 6.6.1

[jurajmajerik]

Problem

Forgot to bump version to 6.6.1 in my previous PR

[greptile-apps]

Greptile Summary

This PR performs a version bump from 6.6.0 to 6.6.1 in the PostHog Python SDK. The changes include updating the version string in posthog/version.py and adding a corresponding entry to CHANGELOG.md documenting a bug fix that prevents a NoneType error when group_properties is None.

This is a standard patch release following semantic versioning conventions. The version bump addresses a defensive programming issue where the SDK would previously crash with a 'NoneType' object has no attribute 'get' error when the group_properties parameter was passed as None instead of a dictionary. The fix was implemented in PR #312 but the version bump was inadvertently missed, making this PR a housekeeping task to properly release the fix.

The changes integrate well with the existing codebase structure - the version file follows the established pattern with a simple VERSION constant, and the CHANGELOG entry maintains consistency with the project's documentation standards by including the version number, date, and descriptive fix entry.

Confidence score: 5/5

• This PR is safe to merge with minimal risk as it contains only version metadata updates
• Score reflects the straightforward nature of version bumps and proper documentation practices
• No files require special attention as both changes are standard administrative updates

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