Remove use of rsyncdir

Fixes: #557

Author: ssbarnea

src/pytest_cov/engine.py

@@ -260,7 +260,9 @@ def start(self):
 
         # Ensure coverage rc file rsynced if appropriate.
         if self.cov_config and os.path.exists(self.cov_config):
-            self.config.option.rsyncdir.append(self.cov_config)
+            # rsyncdir is going away in pytest-xdist 4.0, already deprecated
+            if hasattr(self.config.option, 'rsyncdir'):
+                self.config.option.rsyncdir.append(self.cov_config)
 
         self.cov = coverage.Coverage(source=self.cov_source,
                                      branch=self.cov_branch,