pylint: Fix unnecessary-dict-index-lookup

Pylint 2.9 introduced new check:
> Emitted when iterating over the dictionary items (key-item pairs) and
accessing the value by index lookup. The value can be accessed directly
instead.

Note: in Python3 removing from dict during an iteration is not
possible even. For example,
```
cat a.py
d = {"a": 1}

for k, v in d.items():
    if v is not None:
        del d[k]

python3 a.py
Traceback (most recent call last):
  File "/usr/src/RPM/BUILD/freeipa/a.py", line 3, in <module>
    for k, v in d.items():
RuntimeError: dictionary changed size during iteration
```

Fixes: https://pagure.io/freeipa/issue/9117
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipatests/test_xmlrpc/tracker/base.py

@@ -155,9 +155,7 @@ class ModificationTracker(BaseTracker):
         result = command()
         self.attrs.update(updates)
         self.attrs.update(expected_updates)
-        for key, value in self.attrs.items():
-            if value is None:
-                del self.attrs[key]
+        self.attrs = {k: v for k, v in self.attrs.items() if v is not None}
 
         self.check_update(
             result,