NanoPlot 1.48 fails because Plotly is missing `create_2d_density`

## NanoPlot 1.48 fails because Plotly is missing `create_2d_density`

Installation of Nanoplot through the conda -c bioconda distribution does not pin the correct package dependencies. The biggest issue is plotly (see below), but also using free-threaded versions of python (e.g., >3.14) will have a conflict with pysam.
The report below is for plotly 7.0.0 breaking Nanoplot, but the real issue is installing through conda doesn't follow pinned packages.


### Description

NanoPlot fails while generating its plots with the following error:

```text
AttributeError: module 'plotly.figure_factory' has no attribute 'create_2d_density'
```

The failure occurs in NanoPlot's plotting code when it calls:

```python
plotly.figure_factory.create_2d_density(...)
```

### Environment

Failing environment:

```text
Python: 3.13.15
NanoPlot: 1.48.0
Plotly: 7.0.0
```

Check:

```python
import plotly.figure_factory as ff

print(hasattr(ff, "create_2d_density"))
```

Output:

```text
False
```

### Comparison

The same input works with:

```text
Python: 3.13.15
NanoPlot: 1.47.2
Plotly: 6.9.0
```

In that environment:

```text
create_2d_density: True
```

### NanoPlot failure message  

Usage
```
NanoPlot \
    --barcoded \
    --huge \
    --threads 24 \
    --tsv_stats \
    --only-report \
    --prefix "test_" \
    --summary "sequencing_summary_.txt" \
    --outdir "tmp_stats"
```

NanoPlot reports:

```text
If you read this then NanoPlot 1.48.0 has crashed :-(
Please try updating NanoPlot and see if that helps...
```

The traceback ends with:

```text
AttributeError: module 'plotly.figure_factory' has no attribute 'create_2d_density'
```

### Expected behavior

NanoPlot should either:

- work with the current Plotly release, or
- specify a compatible Plotly version constraint.

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NanoPlot 1.48 fails because Plotly is missing `create_2d_density` #442