text: Title: `SklearnTransformerWrapper` fails when FunctionTransformer used with non-numerical columns
  Body: The `SklearnTransformerWrapper` will throw an error if I try to use it for non-numerical columns, even though some scikit-learn transformers are able to handle non-numerical columns.
  
  Consider the following example:
  ```python
  import numpy as np, pandas as pd
  from sklearn.preprocessing import FunctionTransformer
  from feature_engine.wrappers import SklearnTransformerWrapper
  
  df = pd.DataFrame({
      "col1" : ["1", "2", "3"],
      "col2" : ["a", "b", "c"]
  })
  
  pipeline = SklearnTransformerWrapper(
      FunctionTransformer(lambda x: x.astype(np.float64)),
      variables=["col1"]
  )
  pipeline.fit(df)
  ```
  ```
  TypeError                                 Traceback (most recent call last)
  <ipython-input-1-88807bab80e9> in <module>
       12     variables=["col1"]
       13 )
  ---> 14 pipeline.fit(df)
  
  ~/anaconda3/envs/py3/lib/python3.9/site-packages/feature_engine/wrappers/wrappers.py in fit(self, X, y)
      122 
      123         else:
  --> 124             self.variables_ = _find_or_check_numerical_variables(X, self.variables)
      125 
      126         self.transformer_.fit(X[self.variables_], y)
  
  ~/anaconda3/envs/py3/lib/python3.9/site-packages/feature_engine/variable_manipulation.py in _find_or_check_numerical_variables(X, variables)
       75         # check that user entered variables are of type numerical
       76         if any(X[variables].select_dtypes(exclude="number").columns):
  ---> 77             raise TypeError(
       78                 "Some of the variables are not numerical. Please cast them as "
       79                 "numerical before using this transformer"
  
  TypeError: Some of the variables are not numerical. Please cast them as numerical before using this transformer
  ```