Improve write_report, compat using @singledispatch

[khaeru]

Use @functools.singledispatch() to simplify these operators and make them more easily extensible.

Housekeeping

• Set nitpicky = True in Sphinx configuration and address most nitpicks.
  • Includes downstream nitpicks in ixmp and message_ix docs where genno docstrings/signatures are picked up in autosummaries. Thus supersedes and closes #94.
• Add missing items to API documentation: exceptions.
• Update mypy to 1.7.0.

PR checklist

• [x] Checks all ✅
• [x] Update documentation
• [x] Update doc/whatsnew.rst

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (4812c64) to head (409f29a). Report is 173 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #108 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 48 48 Lines 4043 4102 +59 ========================================= + Hits 4043 4102 +59 ``` | [Files](https://app.codecov.io/gh/khaeru/genno/pull/108?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto) | Coverage Δ | | |---|---|---| | [genno/caching.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcaching.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY2FjaGluZy5weQ==) | `100.00% <ø> (ø)` | | | [genno/compat/graphviz.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcompat%2Fgraphviz.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29tcGF0L2dyYXBodml6LnB5) | `100.00% <ø> (ø)` | | | [genno/compat/plotnine/plot.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcompat%2Fplotnine%2Fplot.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29tcGF0L3Bsb3RuaW5lL3Bsb3QucHk=) | `100.00% <100.00%> (ø)` | | | [genno/compat/pyam/operator.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcompat%2Fpyam%2Foperator.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29tcGF0L3B5YW0vb3BlcmF0b3IucHk=) | `100.00% <100.00%> (ø)` | | | [genno/compat/sdmx.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcompat%2Fsdmx.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29tcGF0L3NkbXgucHk=) | `100.00% <100.00%> (ø)` | | | [genno/compat/xarray.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcompat%2Fxarray.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29tcGF0L3hhcnJheS5weQ==) | `100.00% <ø> (ø)` | | | [genno/config.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fconfig.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29uZmlnLnB5) | `100.00% <ø> (ø)` | | | [genno/core/attrseries.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcore%2Fattrseries.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29yZS9hdHRyc2VyaWVzLnB5) | `100.00% <ø> (ø)` | | | [genno/core/computer.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcore%2Fcomputer.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29yZS9jb21wdXRlci5weQ==) | `100.00% <100.00%> (ø)` | | | [genno/core/describe.py](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree&filepath=genno%2Fcore%2Fdescribe.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto#diff-Z2Vubm8vY29yZS9kZXNjcmliZS5weQ==) | `100.00% <ø> (ø)` | | | ... and [11 more](https://app.codecov.io/gh/khaeru/genno/pull/108?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Paul+Natsuo+Kishimoto) | |