Update to Python 3.10

[danuker]

Currently, there is an error on Py3.10:

_______________________________________________________________________________________________________________ ERROR collecting tests/test_regressor.py ________________________________________________________________________________________________________________
ImportError while importing test module '/home/dan/projects/symreg/tests/test_regressor.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_regressor.py:6: in <module>
    import symreg.ga
symreg/__init__.py:1: in <module>
    from .regressor import Regressor
symreg/regressor.py:4: in <module>
    from symreg.ga import GA
symreg/ga.py:6: in <module>
    from .nsgaii import nsgaii_cull
symreg/nsgaii.py:4: in <module>
    from orderedset import OrderedSet
../../.local/lib/python3.10/site-packages/orderedset/__init__.py:5: in <module>
    from ._orderedset import OrderedSet
E   ImportError: /home/dan/.local/lib/python3.10/site-packages/orderedset/_orderedset.cpython-310-x86_64-linux-gnu.so: undefined symbol: _PyGen_Send

Remove orderedset as a dependency, and use own implementation.