feat(pygal): implement contour-basic (#5333)

## Implementation: `contour-basic` - python/pygal

Implements the **python/pygal** version of `contour-basic`.

**File:** `plots/contour-basic/implementations/python/pygal.py`

**Parent Issue:** #855

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/24867578429)*

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