fix numpy deprecations

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2e29414) 77.27% compared to head (97294f4) 77.27%.

:exclamation: Current head 97294f4 differs from pull request most recent head 8cf16ea. Consider uploading reports for the commit 8cf16ea to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1039 +/- ## ======================================= Coverage 77.27% 77.27% ======================================= Files 200 200 Lines 11308 11308 Branches 1484 1484 ======================================= Hits 8738 8738 Misses 2351 2351 Partials 219 219 ``` | [Impacted Files](https://app.codecov.io/gh/rasbt/mlxtend/pull/1039?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/evaluate/bias\_variance\_decomp.py](https://app.codecov.io/gh/rasbt/mlxtend/pull/1039?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9iaWFzX3ZhcmlhbmNlX2RlY29tcC5weQ==) | `68.88% <100.00%> (ø)` | | | [mlxtend/evaluate/holdout.py](https://app.codecov.io/gh/rasbt/mlxtend/pull/1039?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9ob2xkb3V0LnB5) | `92.85% <100.00%> (ø)` | | | [mlxtend/evaluate/tests/test\_holdout.py](https://app.codecov.io/gh/rasbt/mlxtend/pull/1039?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS90ZXN0cy90ZXN0X2hvbGRvdXQucHk=) | `93.10% <100.00%> (+0.12%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/rasbt/mlxtend/pull/1039/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

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