chore(find-slow-to-highlight-files.py): be explicit about using python3

In many distros, `python` no longer leads to anything, and instead
`python2` or `python3` need to be explicitly run.

tests/scripts/find-slow-to-highlight-files.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # This script goes through all languages that are supported by 'bat'. For each
 # language, it loops over the correspoinding file extensions and searches a