Validate PEP-8 in docstring examples
At the moment we are automatically validating PEP-8 in our code (using `flake8 .` in the CI), and we also have the script `scripts/validate_docstrings.py` that reports errors for different formatting errors in our docstrings, and it also runs the tests to see if they work and generate the expected input. But we are not validating whether the examples follow PEP-8.

In the next example:
```
def head():
    """
    Return the first n rows of the Series.

    Examples
    --------
    >>> s=pd.Series([1, 2, 3, 4])
    >>> s.head(2)
    0    1
    1    2
    dtype: int64
```

Everything will be reported as correct, because the formatting of the docstring is the expected, and when running the examples, they work and they generated the presented output.

According to PEP-8, there should be spaces around the `=` in an assignment. So, we should have `s = pd.Series([1, 2, 3, 4])` instead of `s=pd.Series([1, 2, 3, 4])`. So, the validation should report it as an error.

What we need to do is:
- Add a method in `scripts/validate_docstrings.py` that obtains the code in the examples (and if possible their line in the code to be presented to the user if there are errors). For what I know, the module `doctest` in the Python standard library provides a way to extract the code.
- Add another method that given some code, returns possible PEP-8 errors. We currently have `pyflakes` as a dependency, so using it is probably the best option. But if it's better to use `pycodestyle` or something else, that could be an option.
- Use these methods in the validation, so errors are reported as the rest of the errors from the validation
- Add unit tests for the implemented features in `scripts/tests/test_validate_docstrings.py`
