Revert python statements that are incompatible with Python 3.6

Reviewer's Guide by Sourcery

This PR reverts Python 3.8+ syntax (specifically the walrus operator ':=') to make the code compatible with Python 3.6, which is needed for Raspberry Pi 32-bit builds. The changes primarily involve restructuring conditional assignments into separate statements.

Architecture diagram for new CI workflow

graph TD;
    A[GitHub Actions] -->|Triggers| B[Unittest Workflow];
    B -->|Runs on| C[Windows 2019];
    B -->|Python Version| D[Python 3.6];
    B --> E[Checkout Code];
    B --> F[Setup Python];
    B --> G[Cache Python Environment];
    B --> H[Install Dependencies];
    B --> I[List Environment];
    B --> J[Run Unittests];
    subgraph Workflow Steps
        E --> F --> G --> H --> I --> J
    end

Class diagram for updated conditional assignments

classDiagram
    class OpCut {
        +add_reference(node, pos, kwargs)
        -valid_node_for_reference(node)
        -_children
    }
    class OpEngrave {
        +add_reference(node, pos, kwargs)
        -valid_node_for_reference(node)
        -_children
    }
    class ImageProperty {
        +update_image()
        +calculate_contours()
        -node
        -matrix
        -parameters
    }
    class Wxmtree {
        +refresh_tree(node, level, source)
        -elements
        -wxtree
    }
    class Wxutils {
        +set_color_according_to_theme(control, background, foreground)
    }
    note for OpCut "Reverted walrus operator in add_reference method"
    note for OpEngrave "Reverted walrus operator in add_reference method"
    note for ImageProperty "Reverted walrus operator in update_image and calculate_contours methods"
    note for Wxmtree "Reverted walrus operator in refresh_tree method"
    note for Wxutils "Reverted walrus operator in set_color_according_to_theme method"

File-Level Changes

Change	Details	Files
Replace walrus operator assignments with traditional assignment statements	Split walrus operator assignments into separate variable assignment and conditional check Convert inline assignments in if-statements to two-line statements Maintain logical equivalence while using Python 3.6 compatible syntax	`meerk40t/core/node/op_cut.py` `meerk40t/core/node/op_engrave.py` `meerk40t/gui/propertypanels/imageproperty.py` `meerk40t/gui/wxmtree.py` `meerk40t/gui/wxutils.py`
Add Python 3.6 specific CI workflow	Create new GitHub Actions workflow for Python 3.6 testing Configure workflow to run on Windows 2019 with Python 3.6 Set up caching and dependency installation for the test environment	`.github/workflows/unittest-py36.yml`

Tips and commands

#### Interacting with Sourcery - **Trigger a new review:** Comment `@sourcery-ai review` on the pull request. - **Continue discussions:** Reply directly to Sourcery's review comments. - **Generate a GitHub issue from a review comment:** Ask Sourcery to create an issue from a review comment by replying to it. - **Generate a pull request title:** Write `@sourcery-ai` anywhere in the pull request title to generate a title at any time. - **Generate a pull request summary:** Write `@sourcery-ai summary` anywhere in the pull request body to generate a PR summary at any time. You can also use this command to specify where the summary should be inserted. #### Customizing Your Experience Access your [dashboard](https://app.sourcery.ai) to: - Enable or disable review features such as the Sourcery-generated pull request summary, the reviewer's guide, and others. - Change the review language. - Add, remove or edit custom review instructions. - Adjust other review settings. #### Getting Help - [Contact our support team](mailto:support@sourcery.ai) for questions or feedback. - Visit our [documentation](https://docs.sourcery.ai) for detailed guides and information. - Keep in touch with the Sourcery team by following us on [X/Twitter](https://x.com/SourceryAI), [LinkedIn](https://www.linkedin.com/company/sourcery-ai/) or [GitHub](https://github.com/sourcery-ai).

meerk40t / meerk40t