cap pylint CPUs in qt/

After 4 cores, improvements are tiny, and soon start going backwards,
presumably due to the overhead of importing PyQt into each worker.

qt/tests/run_pylint.py

@@ -2,6 +2,7 @@ import os
 import subprocess
 import sys
 import PyQt5
+import multiprocessing
 
 from pylint.lint import Run
 
@@ -15,7 +16,7 @@ if __name__ == "__main__":
             "--rcfile",
             ini,
             "-j",
-            "0",
+            str(min(4, multiprocessing.cpu_count())),
             "-v",
         ]
     )