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

@@ -33,7 +33,7 @@ This is the main executable, a simple frontend to set up the PYTHONPATH
 and call immediately the command line interface scripts
 """
 
-import importlib
+from importlib import util
 import os
 import sys
 
@@ -48,7 +48,7 @@ if sys.version_info[0] != 3 or sys.version_info[1] <= 6:
     )
 
 # Checking that the 'six' package is present
-if not importlib.util.find_spec("six"):
+if not util.find_spec("six"):
     sys.exit(
         'The python "six" module is not found on your system and it is required for MadAnalysis 5 for '
         + "a question of Python 2/3 compatibility. Please install it with the following command:\n"

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):