Do not depend on pytest_run_parallel.

--- cffi-2.0.0/testing/conftest.py.orig
+++ cffi-2.0.0/testing/conftest.py
@@ -3,11 +3,7 @@
 
 from ctypes import util
 
-try:
-    import pytest_run_parallel  # noqa:F401
-    PARALLEL_RUN_AVAILABLE = True
-except Exception:
-    PARALLEL_RUN_AVAILABLE = False
+PARALLEL_RUN_AVAILABLE = False
 
 def pytest_configure(config):
     # Don't warn for `pytest-run-parallel` markers if they're not available