Use an explicit encoding when opening `os.devnul`

Although slightly pointless, this avoids an `EncodingWarning` in Python 3.10
or greater when running under the `-X warn_default_encoding` flag.

src/_pytest/capture.py

@@ -381,7 +381,7 @@ class FDCaptureBinary:
         self.targetfd_save = os.dup(targetfd)
 
         if targetfd == 0:
-            self.tmpfile = open(os.devnull)
+            self.tmpfile = open(os.devnull, encoding="utf-8")
             self.syscapture = SysCapture(targetfd)
         else:
             self.tmpfile = EncodedFile(