pylint: Teach Pylint how to handle request.context

With Astroid change [0] a inference for builtin containers
was improved. This means that all the elements of such containers
will be inferred if they are not Python constants (previously
ignored).

This change introduces several issues, one of them is a volatile
error exposed at multi-job Pylinting, but could be guaranteed
produced at single-job mode as:

```
PYTHONPATH=. /usr/bin/python3 -m pylint --rcfile=./pylintrc \
    --load-plugins pylint_plugins ipaserver/plugins/dns.py ipalib/request.py

ipalib/request.py:76: [E1101(no-member), destroy_context] Instance of 'bool' has no 'disconnect' member)

-----------------------------------
Your code has been rated at 9.97/10
```

Or even adding 'context.some_attr = True' into ipalib/request.py.
It's should be treated as no one member of `context`'s attrs is a
`Connection` instance and has `destroy_context` member.

To tell Pylint that there are such members the corresponding
transformation is added.

[0] https://github.com/PyCQA/astroid/commit/79d5a3a7

Related: https://pagure.io/freeipa/issue/8116
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Christian Heimes <cheimes@redhat.com>

pylint_plugins.py

@@ -284,6 +284,18 @@ register_module_extender(MANAGER, 'ipaplatform.tasks',
                          ipaplatform_tasks_transform)
 
 
+def ipalib_request_transform():
+    """ipalib.request.context attribute
+    """
+    return AstroidBuilder(MANAGER).string_build(textwrap.dedent('''
+    from ipalib.request import context
+    context._pylint_attr = Connection("_pylint", lambda: None)
+    '''))
+
+
+register_module_extender(MANAGER, 'ipalib.request', ipalib_request_transform)
+
+
 class IPAChecker(BaseChecker):
     __implements__ = IAstroidChecker