[Fix] Public accessibility of EasyPostTimeInTransitData class

[nwithan8]

Description

EasyPostTimeInTransitData was not explicitly marked as public, making it inaccessible to end-users.

Closes #304

Testing

• Unit test updated to confirm class/property of EstimatedDeliveryDate is accessible

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)