fix python3.8 typehints

[casperdcl]

Due to Algorithm.run(callbacks) typehint list[...] introduced in #1659

From https://anvil.softeng-support.ac.uk/jenkins/job/CILsingle/job/CCPi-Framework-dev (related #1598, #1560):

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ERROR: test_ISTA_algorithm (unittest.loader._FailedTest)
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ImportError: Failed to import test module: test_ISTA_algorithm
Traceback (most recent call last):
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\_test_env\lib\unittest\loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\_test_env\lib\unittest\loader.py", line 377, in _get_module_from_ne
    __import__(name)
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\test_tmp\Wrappers\Python\test\test_ISTA_algorithm.py", line 21, in>
    from cil.optimisation.algorithms import ISTA
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\_test_env\lib\site-packages\cil\optimisation\algorithms\__init__.p>
    from .Algorithm import Algorithm
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\_test_env\lib\site-packages\cil\optimisation\algorithms\Algorithm.>
    class Algorithm:
  File "C:\Users\gemst\miniconda3\conda-bld\cil_1708105222305\_test_env\lib\site-packages\cil\optimisation\algorithms\Algorithm.m
    def run(self, iterations=None, callbacks: Optional[list[Callback]]=None, verbose=1, **kwargs):
TypeError: 'type' object is not subscriptable