Refactor RestClient to avoid creating circular references

See #238 for background.

Previously the RestClient inherited from an AsyncToSerialHelper utility class that provided methods for calling / wrapping sync and async functions synchronously. Due to the way I wrote the AsyncToSerialHelper class, it was possible to add an async function to the event loop that contained a reference to both the AsyncToSerialHelper instance and whatever coroutine was going to run. As a result if the coroutine had a longer lifetime than the AsyncToSerialHelper instance, the gc could never collect the AsyncToSerialHelper instance. Crucially this meant the AsyncToSerialHelper instance's finalizer method, __del__ would never run any tear down logic. To avoid this, the functionality of AsyncToSerialHelper was refactored into a module of functions.

fixes #238

`hydrotools._restclient` [3.1.0] - 2023-10-25

Removals

RestClient.close finalizers were removed as they are no longer necessary.

Changes

Loosen _restclient's aiohttp version dependency.
Functionality of AsyncToSerialHelper was refactored into a module of functions. To avoid creation of circular references.

Checklist

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[x] Any change in functionality is tested
[x] New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
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NOAA-OWP / hydrotools