fix: add missing imports so that dataframes can be written to ttrees

[ariostas]

There was a missing import, which caused dataframes not not be writable to TTrees. (Same applied to numpy.recarray.)

[ariostas]

@ianna do you think it's worth having a hotfix release for this? I'm not sure how this wasn't caught by the CI. We do have tests that write simple dataframes, but somehow the import was made somewhere else so it looked fine.

uproot5/tests/test_0412_write_multidimensional_numpy_to_ttree.py

Lines 148 to 158 in c94a38f

	def test_pandas(tmp_path):
	pandas = pytest.importorskip("pandas")
	newfile = os.path.join(tmp_path, "newfile.root")
	df1 = pandas.DataFrame({"x": [1, 2, 3], "y": [1.1, 2.2, 3.3]})
	df2 = pandas.DataFrame({"x": [4, 5, 6], "y": [4.4, 5.5, 6.6]})
	with uproot.recreate(newfile, compression=None) as fout:
	fout.mktree("tree", df1)
	fout["tree"].extend(df2)