Fix x and y axis naming in dendrogram trace

[Brainor]

It's a very simple bug, so I don't think it needs any additional tests or even deserves a changelog entry 😢

plotly.py/packages/python/plotly/plotly/figure_factory/_dendrogram.py

Lines 380 to 391 in 69c20f7

	try:
	x_index = int(self.xaxis[-1])
	except ValueError:
	x_index = ""
	try:
	y_index = int(self.yaxis[-1])
	except ValueError:
	y_index = ""
	trace["xaxis"] = "x" + x_index
	trace["yaxis"] = "y" + y_index

If x_index is int from line 381, line 390 will cause TypeError: can only concatenate str (not "int") to str from concatenating str("x") and int(3).
The error could accur when someone tries to create a dendrogram plot in a subplot in which the xaxis's name is not xaxis but xaxis3.

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• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[alexcjohnson]

💃 Very nice, thanks. I do think we could include a changelog entry if you'd like to add one, but you're right we can do without extra tests.