Adjust for pyam 1.7.0

[khaeru]

pyam 1.7.0 was released on 2022-12-19. Per IAMconsortium/pyam#708, specifically here, keyword arguments to IamDataFrame are directly fed to pandas.DataFrame.to_excel(). (See also the blame for this method. It appears at some point pyam forced engine="openpyxl" and accepted but ignored the keyword arguments.)

This causes failures in genno.compat.pyam.write_report(), e.g. here:

 genno/compat/pyam/computations.py:109: in write_report
    obj.to_excel(path, merge_cells=False)
/opt/hostedtoolcache/Python/3.10.9/x64/lib/python3.10/site-packages/pyam/core.py:2382: in to_excel
    excel_writer = pd.ExcelWriter(excel_writer, **kwargs)

(snip)

>       self._book = Workbook(self._handles.handle, **engine_kwargs)
E       TypeError: Workbook.__init__() got an unexpected keyword argument 'merge_cells'

/opt/hostedtoolcache/Python/3.10.9/x64/lib/python3.10/site-packages/pandas/io/excel/_xlsxwriter.py:216: TypeError

This is because pyam is now allowing pandas to select xlsxwriter as the engine, and the merge_cells keyword argument is not understood by this engine.

The fix is likely to (a) remove and (b) specify a minimum version of pyam to avoid the need for genno to handle the shift(s) in behaviour.

[khaeru]

Closed by #78.