TST: Dynamically use doctest_namespace only if running the doctest
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

Will probably speed up the test suite. Example locally with running `pandas/tests/window/moments`

Main
```
======================================================================= slowest 30 durations =======================================================================
0.07s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data4-True-False-0]
0.06s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_std[all_data9-False-False-2-False]
0.06s teardown pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data4-False-True-2]
0.05s teardown pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_var_debiasing_factors[all_data17-rolling_consistency_cases1-False]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data3-False-False-2]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data6-False-False-0]
0.03s setup    pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data3-False-True-0]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data11-False-False-2]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data3-False-False-0]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data5-False-False-2]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data7-True-True-0]
0.03s setup    pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data5-True-False-2]
0.03s setup    pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data6-False-False-0]
0.03s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data11-False-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data6-True-True-2]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data9-True-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data11-True-True-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data7-True-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data3-False-True-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data9-False-True-0]
0.02s setup    pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data9-False-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data8-True-True-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data12-True-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data9-True-False-2]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data10-True-False-0]
0.02s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data8-False-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data8-False-False-2]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data10-False-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data5-True-True-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data10-True-True-0]
```

This PR
```
======================================== slowest 30 durations =======================================================================
0.01s call     pandas/tests/window/moments/test_moments_consistency_expanding.py::test_expanding_consistency_var_std_cov[all_data13-2-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_expanding.py::test_expanding_consistency_var_std_cov[all_data11-2-1]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_var_debiasing_factors[all_data9-True-True-2]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data6-True-False-2-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_var_debiasing_factors[all_data9-False-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_expanding.py::test_expanding_consistency_var_std_cov[all_data16-2-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_series_cov_corr[series_data1-rolling_consistency_cases1-True-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_series_cov_corr[series_data5-rolling_consistency_cases0-True-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_series_cov_corr[series_data3-rolling_consistency_cases0-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_var_debiasing_factors[all_data12-False-True-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data7-False-True-2-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data2-False-True-0-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data4-False-True-2-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_var_std_cov[all_data17-rolling_consistency_cases1-True-1]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_mean[all_data4-False-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data5-False-True-0-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_var_std_cov[all_data9-rolling_consistency_cases0-True-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data6-False-False-0-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_expanding.py::test_expanding_consistency_var_std_cov[all_data14-2-1]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_var_debiasing_factors[all_data9-rolling_consistency_cases1-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_rolling.py::test_rolling_consistency_series_cov_corr[series_data6-rolling_consistency_cases1-False-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_var_debiasing_factors[all_data13-True-False-2]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_std[all_data14-False-True-0-False]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_std[all_data16-True-False-2-True]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_series_cov_corr[series_data5-True-True-0-False]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_std[all_data9-False-True-0-False]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_moments_consistency_var_constant[consistent_data3-True-True-0-False]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_std[all_data9-True-True-0-False]
0.01s call     pandas/tests/window/moments/test_moments_consistency_expanding.py::test_expanding_consistency_var_std_cov[all_data6-2-0]
0.01s call     pandas/tests/window/moments/test_moments_consistency_ewm.py::test_ewm_consistency_var_debiasing_factors[all_data10-False-True-2]
```
cc @jreback green and should speed up the CI tests backport? Hmm might be a good idea but not critical since this is not a CI-configuration change per se.  let's try @meeseeksdev backport 1.4.x Something went wrong ... Please have a look at my logs.