Use assertEqual() in test_workertrial to fix PyPy3

Use assertEqual() rather than assertIdentical() to compare strings
in twisted.trial._dist.test.test_workertrial to fix the test failure
on PyPy3.

Fixes: ticket:10302

src/twisted/trial/_dist/test/test_workertrial.py

@@ -69,7 +69,7 @@ def fdopen(self, fd, mode=None):
         the stdin fd and C{self.writeStream} for the stdout fd.
         """
         if fd == _WORKER_AMP_STDIN:
-            self.assertIdentical("rb", mode)
+            self.assertEqual("rb", mode)
             return self.readStream
         elif fd == _WORKER_AMP_STDOUT:
             self.assertEqual("wb", mode)

src/twisted/trial/newsfragments/10302.bugfix

@@ -0,0 +1 @@
+_dist.test.test_workertrial now correctly compare strings via assertEqual() and pass on PyPy3