Disable irrelevant pylint errors

Errors were:

```
************* Module stbt_rig
stbt_rig.py:1237:4: [W0223(abstract-method), StbtCollector] Method 'get_closest_marker' is abstract in class 'Node' but is not overridden
stbt_rig.py:1237:4: [W0223(abstract-method), StbtCollector] Method 'gethookproxy' is abstract in class 'FSCollector' but is not overridden
stbt_rig.py:1237:4: [W0223(abstract-method), StbtCollector] Method 'isinitpath' is abstract in class 'FSCollector' but is not overridden
stbt_rig.py:1257:4: [W0223(abstract-method), StbtRemoteTest] Method 'get_closest_marker' is abstract in class 'Node' but is not overridden
```

The `get_closest_marker` is a pylint false positive because it's defined
multiple times in `class Node`:

```python
class Node(metaclass=NodeMeta):
    ...
    @overload
    def get_closest_marker(self, name: str) -> Optional[Mark]:
        raise NotImplementedError()

    @overload  # noqa: F811
    def get_closest_marker(self, name: str, default: Mark) -> Mark:  # noqa: F811
        raise NotImplementedError()

    def get_closest_marker(  # noqa: F811
        self, name: str, default: Optional[Mark] = None
    ) -> Optional[Mark]:
        """return the first marker matching the name, from closest (for example function) to farther level (for example
        module level).

        :param default: fallback return value of no marker was found
        :param name: name to filter by
        """
        return next(self.iter_markers(name=name), default)
```

The other two don't need to be inherited because they'll never be called:
pytest-dev/pytest#7591 .  This is fixed in
pytest 6.1: pytest-dev/pytest#7648

stbt_rig.py

@@ -1235,6 +1235,7 @@ def pytest_collect_file(path, parent):
 
 
     class StbtCollector(pytest.File):
+        # pylint: disable=abstract-method
         def collect(self):
             with open(self.fspath.strpath) as f:
                 # We implement our own parsing to avoid import stbt ImportErrors
@@ -1255,6 +1256,7 @@ def collect(self):
 
 
     class StbtRemoteTest(pytest.Item):
+        # pylint: disable=abstract-method
         def __init__(self, parent, filename, testname, line_number):
             super(StbtRemoteTest, self).__init__(testname, parent)
             self._filename = filename