LGTM.com - false positive - float is numbers.Number

**Description of the false positive**

I may be missing something here, but the message doesn't make much sense:
> `[ 3 Values ]` of class '`[ 3 Values ]`' may be used in for-loop.`

What are these "`[ 3 Values ]`" about?

Also, note that a `float` **is** a `numbers.Number`:
```python
>>> import numbers
>>> x = 0.5
>>> isinstance(x, numbers.Number)
True
>>>
```
So, since `l1_ratios` is usually initialised to a float as far as I can see, it will be changed into an iterable list, won't it?
```python
if isinstance(l1_ratios, numbers.Number):
l1_ratios = [l1_ratios]
```

**URL to the alert on the project page on LGTM.com**

https://lgtm.com/projects/g/nilearn/nilearn/snapshot/0cd925e33787e277990b524efeba0c8125d04816/files/nilearn/decoding/space_net.py#x4b41318e0293a848:1