Refactor and fix UnityVersion __getitem__

[dv1x3r]

I encountered an issue exporting meshes from a bundle with UnityVersion 5.3p6.

Sample: bundle_5.3p6.zip

Using the following minimal example:

for obj in env.objects:    
    if obj.type.name == "Mesh":
        data = obj.parse_as_object()
        filename = data.m_Name+".obj"
        file_path = os.path.join(export_path, env.file.name, "models", filename)
        with open(file_path, "wt", newline="") as f:
            f.write(data.export())

I get this error:

File ~/.venv/lib/python3.13/site-packages/UnityPy/helpers/MeshHelper.py:165, in MeshHandler.process(self)
    159 if mesh.m_Use16BitIndices is not None:
    160     self.m_Use16BitIndices = bool(mesh.m_Use16BitIndices)
    161 elif (
    162     self.version >= (2017, 4)
    163     or
    164     # version == (2017, 3, 1) & patched - px string
--> 165     self.version[:2] == (2017, 3)
    166     and mesh.m_MeshCompression == 0
    167 ):
    168     self.m_Use16BitIndices = mesh.m_IndexFormat == 0
    169 self.copy_from_mesh()

File ~/.venv/lib/python3.13/site-packages/UnityPy/helpers/UnityVersion.py:91, in UnityVersion.__getitem__(self, idx)
     89 elif idx == 4:
     90     return self.type_number
---> 91 raise IndexError("Invalid UnityVersion index")

IndexError: Invalid UnityVersion index

The current UnityVersion.__getitem__ implementation does not support slicing, and always raises index error.

In this PR I did not change the (2017, 3) condition (I think it would still be better to add explicit parentheses because of the mixed and and or conditions) and instead added slice support.