Drop Python 3.7 support -- EOLed

[codecov[bot]]

Codecov Report

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

Comparison is base (61a7279) 89.16% compared to head (5e96572) 89.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #129 +/- ## ======================================= Coverage 89.16% 89.16% ======================================= Files 24 24 Lines 1181 1181 ======================================= Hits 1053 1053 Misses 128 128 ```

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[yarikoptic]

docs fail is unrelated afaik

Warning, treated as error:
Failed to read intersphinx_mapping[https://docs.python.org/], ignored: SphinxWarning('The pre-Sphinx 1.0 \'intersphinx_mapping\' format is deprecated and will be removed in Sphinx 8. Update to the current format as described in the documentation. Hint: "intersphinx_mapping = {\'<name>\': (\'[https://docs.python.org/\](https://docs.python.org//)', None)}".https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping')
make: *** [Makefile:55: html] Error 2
make: Leaving directory '/home/runner/work/datalad-neuroimaging/datalad-neuroimaging/docs'