feat: convert to async using httpx (v3.0.0)

[mayurrawte]

Summary

Convert the library to async using httpx for better performance in async applications like FastAPI/pynexus.

BREAKING CHANGE: All methods are now async and require await.

Changes

• Replace requests with httpx for async HTTP calls
• All API methods now return coroutines
• Add httpx>=0.24.0 as dependency
• Bump version to 3.0.0

Usage

import asyncio
from ShipthisAPI import ShipthisAPI

async def main():
    client = ShipthisAPI(
        organisation="your_org_id",
        x_api_key="your_api_key"
    )

    await client.connect()
    items = await client.get_list("shipment")
    await client.patch_item("fcl_load", doc_id, {"status": "completed"})

asyncio.run(main())

Test plan

• Verify async methods work correctly
• Test in pynexus environment