Fix PHPStan errors

Author: DonCallisto

phpstan.neon

@@ -5,4 +5,7 @@ parameters:
 	level: 5
 	paths:
 		- src
-		- tests
+		- tests
+	reportUnmatchedIgnoredErrors: false
+	ignoreErrors: #see https://github.com/phpstan/phpstan/issues/1267#issuecomment-552874947 -> @todo remove when sf 3.4 support ends
+	    - "#^Call to an undefined static method #"

src/Process/ProcessorCounter.php

@@ -55,6 +55,7 @@ private function readFromProcCPUInfo()
                 $process = Process::fromShellCommandline($executeCommand);
             } else {
                 // Drop when sf 3.4 supports ends
+                /** @phpstan-ignore-next-line */ // @todo remove when sf 3.4 support ends
                 $process = new Process($executeCommand);
             }
             $process->run();