Access ast.Constant.value, not .s -- the latter is gone from Python 3.14+

[hroncok]

Followup for #498

Test failures this fixes:

_____________________________ test_init_good["x"] ______________________________

setting_function_good = <UM.Settings.SettingFunction (0x7f36b42aa200) ="x" >

    def test_init_good(setting_function_good):
        assert setting_function_good is not None
>       assert setting_function_good.isValid()
E       assert False
E        +  where False = isValid()
E        +    where isValid = <UM.Settings.SettingFunction (0x7f36b42aa200) ="x" >.isValid

tests/Settings/TestSettingFunction.py:64: AssertionError
---------------------------- Captured stdout setup -----------------------------
[MainThread] UM.Settings.SettingFunction._safeCompile [80]: Exception in function ('Constant' object has no attribute 's') for setting: "x"
_______________________________ test_call[data1] _______________________________

data = {'code': '"x"', 'result': 'x'}

    @pytest.mark.parametrize("data", test_call_data)
    def test_call(data):
        value_provider = MockValueProvider()
        function = SettingFunction(data["code"])
>       assert function(value_provider) == data["result"]
E       assert None == 'x'
E        +  where None = <UM.Settings.SettingFunction (0x7f36be9e57f0) ="x" >(<tests.Settings.TestSettingFunction.MockValueProvider object at 0x7f36c0f27250>)

tests/Settings/TestSettingFunction.py:115: AssertionError
----------------------------- Captured stdout call -----------------------------
[MainThread] UM.Settings.SettingFunction._safeCompile [80]: Exception in function ('Constant' object has no attribute 's') for setting: "x"
_________________________ test_getUsedSettings[data1] __________________________

data = {'code': '"x"', 'variables': ['x']}

    @pytest.mark.parametrize("data", test_getUsedSettings_data)
    def test_getUsedSettings(data):
        function = SettingFunction(data["code"])
        answer = function.getUsedSettingKeys()
>       assert len(answer) == len(data["variables"])
E       AssertionError: assert 0 == 1
E        +  where 0 = len(frozenset())
E        +  and   1 = len(['x'])

tests/Settings/TestSettingFunction.py:159: AssertionError
----------------------------- Captured stdout call -----------------------------
[MainThread] UM.Settings.SettingFunction._safeCompile [80]: Exception in function ('Constant' object has no attribute 's') for setting: "x"
_________________________ test_getUsedSettings[data7] __________________________

data = {'code': "sqrt('x')", 'variables': ['x']}

    @pytest.mark.parametrize("data", test_getUsedSettings_data)
    def test_getUsedSettings(data):
        function = SettingFunction(data["code"])
        answer = function.getUsedSettingKeys()
>       assert len(answer) == len(data["variables"])
E       AssertionError: assert 0 == 1
E        +  where 0 = len(frozenset())
E        +  and   1 = len(['x'])

tests/Settings/TestSettingFunction.py:159: AssertionError
----------------------------- Captured stdout call -----------------------------
[MainThread] UM.Settings.SettingFunction._safeCompile [80]: Exception in function ('Constant' object has no attribute 's') for setting: sqrt('x')

[github-actions]

Test Results

2 407 tests  ±0   2 392 ✅ ±0   22s ⏱️ -3s
    1 suites ±0      15 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit fd201d1. ± Comparison against base commit 792ab39.