Always enable data_suffix for --cov-append

Setting `data_suffix=True` will also enable `parallel=True` and ensure
the coverage processes do not conflict with each other. Because
pytest-cov always combines the coverage result this is safe to enable.

Author: Urth

src/pytest_cov/engine.py

@@ -218,6 +218,7 @@ def start(self):
 
         self.cov = coverage.Coverage(source=self.cov_source,
                                      branch=self.cov_branch,
+                                     data_suffix=True,
                                      config_file=self.cov_config)
         self.combining_cov = coverage.Coverage(source=self.cov_source,
                                                branch=self.cov_branch,