[BUG] in notebook 07 - pytest not installed by default as dependency

if installing dependencies using requirements.txt then following code section from notebook 07  would throw error about absent pytest:
```python 
from sktime.transformations.series.boxcox import BoxCoxTransformer
from sktime.utils.estimator_checks import check_estimator

res = check_estimator(BoxCoxTransformer)```




