Correct use of either_dict_or_kwargs() in AttrSeries.sel()

[khaeru]

The third argument gives the name of the method, not the argument. This improves the error message shown when the function is miscalled.

Also:

• Bump Python requirement to 3.7, matching xarray >=0.17.
• Advertise support for Python 3.10, although this is pending upstream dependencies like numba and llvmlite. See Include Python version 3.10 and remove 3.6 in CI iiasa/message_ix#532 for closer tracking of these.
• Adjust for pint 0.18.

PR checklist

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

[codecov-commenter]

Codecov Report

Merging #52 (38c8fe8) into main (3bfb54d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         2599      2597    -2     
=========================================
- Hits          2599      2597    -2     

Impacted Files	Coverage Δ
genno/tests/core/test_exceptions.py	100.00% <ø> (ø)
genno/caching.py	100.00% <100.00%> (ø)
genno/core/attrseries.py	100.00% <100.00%> (ø)
genno/testing.py	100.00% <100.00%> (ø)
genno/tests/core/test_computer.py	100.00% <100.00%> (ø)
genno/tests/test_caching.py	100.00% <100.00%> (ø)

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