fix: Add missing importlib.util import

* Without first importing importlib.util an AttributeError will be raised

```
Traceback (most recent call last):
  File "/madanalysis5/./bin/ma5", line 51, in <module>
    if not importlib.util.find_spec("six"):
           ^^^^^^^^^^^^^^
AttributeError: module 'importlib' has no attribute 'util'
```

bin/ma5

@@ -34,6 +34,7 @@ and call immediately the command line interface scripts
 """
 
 import importlib
+import importlib.util
 import os
 import sys
 

doc/releases/changelog-dev.md

@@ -6,6 +6,9 @@
 
 ## Bug fixes
 
+* Add missing `importlib.util` import.
+([#278](https://github.com/MadAnalysis/madanalysis5/pull/278))
+
 ## Contributors
 
 This release contains contributions from (in alphabetical order):