0.21.0 release

[codecov[bot]]

Codecov Report

Base: 77.45% // Head: 77.43% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (6cadd40) compared to base (1f46260). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #972 +/- ## ========================================== - Coverage 77.45% 77.43% -0.02% ========================================== Files 198 198 Lines 11165 11165 Branches 1498 1498 ========================================== - Hits 8648 8646 -2 - Misses 2304 2305 +1 - Partials 213 214 +1 ``` | [Impacted Files](https://codecov.io/gh/rasbt/mlxtend/pull/972?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/\_\_init\_\_.py](https://codecov.io/gh/rasbt/mlxtend/pull/972/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [mlxtend/evaluate/counterfactual.py](https://codecov.io/gh/rasbt/mlxtend/pull/972/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9jb3VudGVyZmFjdHVhbC5weQ==) | `93.10% <0.00%> (-6.90%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

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