LGTM.com - false positive - classmethod() vs. @classmethod

**Description of the false positive**

LGTM.com appears to detect classes with the `@classmethod` decorator:
```python
@classmethod
def from_filename(cls):
[....]
```
but not classes with the subsequent `classmethod`() call:
```python
def from_filename(cls):
[....]

from_filename = classmethod(from_filename)
```

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

https://lgtm.com/projects/g/nilearn/nilearn/snapshot/0cd925e33787e277990b524efeba0c8125d04816/files/nilearn/externals/tempita/__init__.py?sort=name&dir=ASC&mode=heatmap#x264089f243e25194:1