Remove dependency on obsolete third-party package

[claui]

The third-party package future is no longer maintained and considered obsolete due to the end-of-life status of Python 2:

Python 2 reached its end of life in 2020 and the python-future package should no longer be necessary. Use it to help with porting legacy code to Python 3 but don't depend on it for new code.

This PR changes all call sites to use Python 3’s native implementation and removes the future dependency.

Tests

Small self-contained example to prove that the updated code works:

from pygreat import comms

backend = comms.CommsBackend()
core = backend.apis['core']
core.get_available_verbs()

Result:

Traceback (most recent call last):
  File "/home/claudia/Documents/dev/libgreat/test.py", line 5, in <module>
    core.get_available_verbs()
    ~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/pygreat/comms.py", line 1123, in method
    return self.execute_command(verb_number, in_format, out_format, name=name, class_name=class_name,
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            timeout=timeout, max_response_length=max_response_length, encoding=encoding, *arguments)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/pygreat/comms.py", line 1278, in execute_command
    return self.comms_backend.execute_command(self.CLASS_NUMBER, verb, in_format,
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            out_format, *arguments, **kwargs)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/pygreat/comms.py", line 777, in execute_command
    raise outer_exception.with_traceback(e.__traceback__) from None
  File "/usr/lib/python3.13/site-packages/pygreat/comms.py", line 770, in execute_command
    payload = self.pack(in_format, *arguments)
  File "/usr/lib/python3.13/site-packages/pygreat/comms.py", line 581, in pack
    result += struct.pack(subformat, *args_consumed)
              ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: invalid arguments in call to RPC `get_available_verbs`; innner message: pack expected 1 items for packing (got 0); format: <I

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