Adjust .compat.pyam for 1.7

[khaeru]

IAMconsortium/pyam#708 / pyam 1.7.0 changed the behaviour of IamDataFrame.to_excel() by passing extra keyword arguments to pandas.ExcelWriter(), where previously they were discarded.

This broke pyam.computations.write_report(), which used merge_cells=True per the pandas documentation. This argument is not accepted by xlsxwriter, which is forced by pyam.

This PR adjusts.

Also:

• Adjust for mypy no_implicit_optional.
• Adjust invocation of dask.visualize(): pass the Computer instance itself as an argument and provide the __dask_graph__ and __dask_keys__ dunder methods.

PR checklist

• Checks all ✅
• Update documentation N/A
• Update doc/whatsnew.rst

[codecov-commenter]

Codecov Report

Merging #78 (aa016d1) into main (37f78e1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #78   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           39        39           
  Lines         2964      2968    +4     
=========================================
+ Hits          2964      2968    +4     

Impacted Files	Coverage Δ
genno/compat/pyam/computations.py	100.00% <100.00%> (ø)
genno/computations.py	100.00% <100.00%> (ø)
genno/config.py	100.00% <100.00%> (ø)
genno/core/attrseries.py	100.00% <100.00%> (ø)
genno/core/computer.py	100.00% <100.00%> (ø)
genno/core/quantity.py	100.00% <100.00%> (ø)
genno/core/sparsedataarray.py	100.00% <100.00%> (ø)

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