[Fix] version sklearn valid_metrics

[danibene]

Description

This PR aims to fix failing tests related to sklearn.neighbors.KDTree.valid_metrics. See also: #814 (comment)

Proposed Changes

I changed the valid_metrics to valid_metrics() in complexity.utils() according to current documentation: https://scikit-learn.org/stable/modules/generated/sklearn.neighbors.KDTree.html

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• I have read the CONTRIBUTING file.
• My PR is targetted at the dev branch (and not towards the master branch).

[codecov-commenter]

Codecov Report

Patch coverage: 85.81% and project coverage change: +1.11 🎉

Comparison is base (f6d4d2d) 54.12% compared to head (59bf6d2) 55.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #861      +/-   ##
==========================================
+ Coverage   54.12%   55.23%   +1.11%     
==========================================
  Files         295      296       +1     
  Lines       13810    13912     +102     
==========================================
+ Hits         7474     7684     +210     
+ Misses       6336     6228     -108     

Impacted Files	Coverage Δ
neurokit2/eda/eda_process.py	92.00% <ø> (ø)
neurokit2/misc/report.py	94.18% <ø> (+2.32%)	⬆️
neurokit2/ppg/ppg_clean.py	70.58% <33.33%> (ø)
neurokit2/ppg/ppg_methods.py	85.29% <66.66%> (ø)
neurokit2/emg/emg_clean.py	83.33% <75.00%> (+1.51%)	⬆️
neurokit2/events/events_find.py	80.00% <75.00%> (-0.96%)	⬇️
neurokit2/complexity/utils.py	80.00% <83.33%> (ø)
neurokit2/ecg/ecg_clean.py	86.95% <85.71%> (+0.19%)	⬆️
neurokit2/signal/signal_filter.py	70.90% <86.66%> (+2.28%)	⬆️
neurokit2/emg/emg_methods.py	90.00% <90.00%> (ø)
... and 4 more

... and 8 files with indirect coverage changes

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[DominiqueMakowski]

brilliant! as always feel free to merge all these PRs :)