Clean up u' prefixes and py2 bytes conversions

src/_pytest/pathlib.py

@@ -134,9 +134,7 @@ def create_cleanup_lock(p):
             raise
     else:
         pid = os.getpid()
-        spid = str(pid)
-        if not isinstance(spid, bytes):
-            spid = spid.encode("ascii")
+        spid = str(pid).encode()
         os.write(fd, spid)
         os.close(fd)
         if not lock_path.is_file():