Make regular expressions in test_tasks.py raw strings. (GH-8759)

Follow up to bpo-34270.

Fixes:
```
Lib/test/test_asyncio/test_tasks.py:330: DeprecationWarning: invalid escape sequence \d
  match1 = re.match("^<Task pending name='Task-(\d+)'", repr(t1))
Lib/test/test_asyncio/test_tasks.py:332: DeprecationWarning: invalid escape sequence \d
  match2 = re.match("^<Task pending name='Task-(\d+)'", repr(t2))
```

Lib/test/test_asyncio/test_tasks.py

@@ -327,9 +327,9 @@ class BaseTaskTests:
         t2 = self.new_task(self.loop, notmuch(), None)
         self.assertNotEqual(repr(t1), repr(t2))
 
-        match1 = re.match("^<Task pending name='Task-(\d+)'", repr(t1))
+        match1 = re.match(r"^<Task pending name='Task-(\d+)'", repr(t1))
         self.assertIsNotNone(match1)
-        match2 = re.match("^<Task pending name='Task-(\d+)'", repr(t2))
+        match2 = re.match(r"^<Task pending name='Task-(\d+)'", repr(t2))
         self.assertIsNotNone(match2)
 
         # Autogenerated task names should have monotonically increasing numbers