Make use new pyxform API

[lindsay-stevens]

Re: #81 / https://github.com/XLSForm/pyxform/pull/712/files

Maybe XLSForm-online could use the new code? E.g. in views.py

from pyxform.xls2xform import convert

converted = convert(xlsform=request.FILES["file"])
with open(xml_path, "w") as f:
    f.write(converted.xform)
if converted.itemsets is not None:
    itemsets_csv = os.path.join(temp_dir, "itemsets.csv")
    with open(itemsets_csv, "w") as f:
        f.write(converted.itemsets)

Warnings is available in converted.warnings.