Remove use of rsyncdir

Fixes: #557

Author: ssbarnea

docs/xdist.rst

@@ -30,7 +30,7 @@ Again but spread over different hosts and different directories::
     pytest --cov=myproj --dist load
             --tx ssh=memedough@host1//chdir=testenv1
             --tx ssh=memedough@host2//chdir=/tmp/testenv2//python=/tmp/env1/bin/python
-            --rsyncdir myproj --rsyncdir tests --rsync examples
+            --rsync examples
             tests/
 
 Shows a terminal report::

src/pytest_cov/engine.py

@@ -248,7 +248,9 @@ 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,