Fix: typing_extensions import causing issues

[kavanase]

Hi Jimmy!
Just a quick fix for something that's causing failures in our doped/ShakeNBreak tests. When importing typing_extensions, one gets a ModuleNotFoundError, as it's not an explicit dependency. I flagged this for pymatgen: materialsproject/pymatgen#3751, fixed in materialsproject/pymatgen#3752, where the from typing_extensions import Self were added to if TYPE_CHECKING: blocks in the last two releases. I've added this fix here now.

Traceback:

ImportError while importing test module '/home/runner/work/doped/doped/tests/test_wyckoff.py'.
Traceback:
/opt/hostedtoolcache/Python/3.11.9/x64/lib/python3.11/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_wyckoff.py:13: in <module>
    from doped.utils.symmetry import get_wyckoff_dict_from_sgn, get_wyckoff_label_and_equiv_coord_list
doped/utils/symmetry.py:11: in <module>
    from pymatgen.analysis.defects.core import DefectType
/opt/hostedtoolcache/Python/3.11.9/x64/lib/python3.11/site-packages/pymatgen/analysis/defects/core.py:18: in <module>
    from typing_extensions import Self
E   ModuleNotFoundError: No module named 'typing_extensions'

GH Actions run link: https://github.com/SMTG-Bham/doped/actions/runs/8951918813/job/24588753603

[jmmshn]

thanks @kavanase !