Do not throw error when getsource fails, fixes #27

Some libraries generate models in runtime, eg. django-simple-history
there is no source code of the models and it doesn't make sense to
parse it because there won't be any ignore comments that we look for

Author: kalekseev

src/extra_checks/ast/source_provider.py

@@ -43,7 +43,7 @@ def __init__(self, obj: Type) -> None:
     def source(self) -> Optional[str]:
         try:
             return textwrap.dedent(inspect.getsource(self._obj))
-        except TypeError:
+        except (TypeError, OSError):
             # TODO: add warning?
             return None