Deprecate axis argument from TrianglePandas.groupby()

[genedan]

Pandas 3.0 removed the axis argument from DataFrame.groupby() the maintainers want you to use transpose instead:

https://pandas.pydata.org/docs/dev/whatsnew/v3.0.0.html#other-removals
pandas-dev/pandas#51203

This leads to this test failing:

chainladder-python/chainladder/core/tests/test_triangle.py

Lines 316 to 327 in 6ea3577

	def test_groupby_axis1(clrd, prism):
	clrd = clrd.sum("origin").sum("development")
	groups = [i.find("Loss") >= 0 for i in clrd.columns]
	assert np.all(
	clrd.to_frame(origin_as_datetime=False).groupby(groups, axis=1).sum()
	== clrd.groupby(groups, axis=1).sum().to_frame(origin_as_datetime=False)
	)
	assert np.all(
	clrd.to_frame(origin_as_datetime=False).groupby("LOB").sum()
	== clrd.groupby("LOB").sum().to_frame(origin_as_datetime=False)
	)
	prism.sum().grain("OYDY")

Unfortunately, I think we'll need to do a bit of rewriting for groupby. The test still seems to be testing a real-life use case, so instead of testing axis=1, it'll test the transpose functionality of the Triangle.