Fix return type in PanderaTransformer::to_python_value()

[danieldanciu]

Return an object of the correct type in the to_python_value of the pandera type transformer. Currently, the returned type is pandas.core.frame.DataFrame, but the declared return type is pandera.typing.DataFrame.
I suspect this is because instantiating pandera.DataFrameSchema returns the former rather than what one would normally expect, which is the latter (the pandera DataFrame).

Closes flyteorg/flyte##5369

Why are the changes needed?

Typeguard is unhappy about Flyte workflows that use Pandera, because the type transformer returns an object of the incorrect type.

What changes were proposed in this pull request?

Simply mark the return type as pandera.typing.DataFrame (which inherits from the currently returned type, so I don't expect any major side-effects).

How was this patch tested?

Created a test workflow, checked with typeguard and everything is now running.

Setup process

Not needed.

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed. I haven't run any tests, as there aren't any directly related tests AFAICT. I am hoping the Gitlab CI/CD will run them for me.
• All commits are signed-off.

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[pingsutw]

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