wxGUI/datacatalog: Replace lambda expression with named Function to fix E731

[arohanajit]

Description

This PR addresses the E731 linting issue in tree.py by replacing a lambda expression with a named function. The change ensures that the code adheres to PEP 8 guidelines, which recommend using a def statement instead of assigning a lambda expression.

Changes Made

• Defined a new function _callback to replace the lambda expression.
• Updated the code to use the _callback function.

Rationale

• Just recreated lambda function with a normal def function as specified in the flake8

[echoix]

To review carefully. Last time I touched lambdas and gui, we ended up needing to revert.

[petrasovaa]

I think this should work, a simple example:

>>> x = 1
>>> def test(x=x):
...     print(x)
... 
>>> test()
1
>>> x = 2
>>> test()
1

Just renaming the function (see the comment) seemed unnecessary, no?

[petrasovaa]

Looks like removing E402 is unrelated, but as long as the checks pass, that's fine I guess, probably some leftover.