Update TypeVar definition for covariant typing in Ref class

[seriaati]

Description

This PR adds covariant=True to the TypeVar definition in Ref.

This is added because otherwise pyright will raise error, example:

Argument of type "Ref[TextField]" cannot be assigned to parameter "ref" of type "Ref[BaseControl] | None" in function "__init__"
  Type "Ref[TextField]" is not assignable to type "Ref[BaseControl] | None"
    "Ref[TextField]" is not assignable to "Ref[BaseControl]"
      Type parameter "T@Ref" is invariant, but "TextField" is not the same as "BaseControl"
    "Ref[TextField]" is not assignable to "None"

Test Code

import flet as ft

ref = ft.Ref[ft.TextField]()
text_field = ft.TextField(ref=ref)

pyright .

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I signed the CLA.
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass locally with my changes
• I have made corresponding changes to the documentation (if applicable)

Screenshots

Additional details

Summary by Sourcery

Bug Fixes:

• Fix type checking errors by marking the Ref generic type parameter as covariant.

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All committers have signed the CLA.

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