build: fix typing of Target.get_option

Which can return any of `ElementaryOptionValues`, but is currently typed
as if it only returns `str | int | bool`.

mesonbuild/build.py

@@ -49,6 +49,7 @@
     from .mesonlib import ExecutableSerialisation, FileMode, FileOrString
     from .modules import ModuleState
     from .mparser import BaseNode
+    from .options import ElementaryOptionValues
 
     GeneratedTypes = T.Union['CustomTarget', 'CustomTargetIndex', 'GeneratedList']
     LibTypes = T.Union['SharedLibrary', 'StaticLibrary', 'CustomTarget', 'CustomTargetIndex']
@@ -679,10 +680,8 @@ def set_option_overrides(self, option_overrides: T.Dict[OptionKey, str]) -> None
     def get_options(self) -> coredata.OptionsView:
         return self.options
 
-    def get_option(self, key: 'OptionKey') -> T.Union[str, int, bool]:
-        # TODO: if it's possible to annotate get_option or validate_option_value
-        # in the future we might be able to remove the cast here
-        return T.cast('T.Union[str, int, bool]', self.options.get_value(key))
+    def get_option(self, key: OptionKey) -> ElementaryOptionValues:
+        return self.options.get_value(key)
 
     @staticmethod
     def parse_overrides(kwargs: T.Dict[str, T.Any]) -> T.Dict[OptionKey, str]: