Exact calculation of effect sizes in pairwise_tukey and pairwise_gameshowell (#328)

* Exact calculation of effect sizes for pairwise functions

#309

* Add link to PR in changelog

* Remove useless import

Author: raphaelvallat

docs/changelog.rst

@@ -31,6 +31,7 @@ We have added the :py:func:`pingouin.ptests` function to calculate a T-test (T-
 - Added customization options to :py:func:`pingouin.plot_rm_corr`, which now takes optional keyword arguments to pass through to :py:func:`seaborn.regplot` and :py:func:`seaborn.scatterplot`. `PR 312 <https://github.com/raphaelvallat/pingouin/pull/312>`_.
 - Changed some plotting functions to increase compatibility with :py:class:`seaborn.FacetGrid`. As explained in `issue 306 <https://github.com/raphaelvallat/pingouin/issues/306>`_, the major change is to generate :py:class:`matplotlib.axes` using default parameters instead of accepting ``fig`` and ``dpi`` keyword arguments. This change applies to :py:func:`pingouin.plot_blandaltman`, :py:func:`pingouin.plot_paired`, :py:func:`pingouin.plot_circmean`, and :py:func:`pingouin.qqplot`. In the future, open a :py:class:`matplotlib.axes` and pass it through using the ``ax`` parameter to use custom figure settings with these functions. Other minor changes include the addition of the ``square`` keyword argument to :py:func`pingouin.plot_circmean` and :py:func`pingouin.qqplot` to ensure equal aspect ratios, and the removal of ``scatter_kws`` as a keyword argument in :py:func:`pingouin.plot_blandaltmann` (now alter the scatter parameters using general ``**kwargs``). `PR 314 <https://github.com/raphaelvallat/pingouin/pull/314>`_.
 - :py:func:`pingouin.normality` does not raise an AssertionError anymore if one of the groups in ``group`` has ≤ 3 samples. `PR 324 <https://github.com/raphaelvallat/pingouin/pull/324>`_.
+- Effect sizes are now calculated using an exact method instead of an approximation based on T-values in :py:func:`pingouin.pairwise_tukey` and :py:func:`pingouin.pairwise_gameshowell`. `PR 328 <https://github.com/raphaelvallat/pingouin/pull/328>`_.
 
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