Assume os.getppid is always available (#2038)

It's always available since 3.2 and tests are already assuming that.

elasticapm/base.py

@@ -374,7 +374,7 @@ def get_service_info(self):
     def get_process_info(self):
         result = {
             "pid": os.getpid(),
-            "ppid": os.getppid() if hasattr(os, "getppid") else None,
+            "ppid": os.getppid(),
             "title": None,  # Note: if we implement this, the value needs to be wrapped with keyword_field
         }
         if self.config.include_process_args: