Python: [BugFix] ReadOnlySkillCollection is not a Generic (#2364)

All (unit) tests are passing.

### Motivation and Context

This change reduces the number of user facing changes and aligns the
type of
ReadOnlySkillCollection with the usage in SK. There is only one type of
`SkillCollection` in SK, so it doesn't make sense to make
`ReadOnlySkillCollection`
a generic. If, in the future, another SkillCollection type is added,
then we might
want to change this to a generic type.

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### Description

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### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

python/tests/unit/test_serialization.py

@@ -115,6 +115,7 @@ def sk_factory() -> t.Callable[[t.Type[_Serializable]], _Serializable]:
             False,
         ),
         FunctionsView: create_functions_view(),
+        ReadOnlySkillCollection: create_skill_collection().read_only_skill_collection,
         SkillCollection: create_skill_collection(),
     }
 
@@ -171,6 +172,7 @@ PYDANTIC_MODELS = [
     ParameterView,
     FunctionView,
     FunctionsView,
+    ReadOnlySkillCollection,
     SkillCollection,
 ]