validation: make line-directive python 3.5 friendly

`ModuleNotFoundError` was introduced in python 3.6 as a child of
`ImportError`.  However, some of the CI hosts use python 3.5.  Use
`ImportError` as the filter type to be compatible with earlier python
versions.

Author: compnerd

utils/line-directive

@@ -63,7 +63,7 @@ def _make_line_map(target_filename, stream=None):
     """
     >>> try:
     ...     from StringIO import StringIO # py2
-    ... except ModuleNotFoundError:
+    ... except ImportError:
     ...     from io import StringIO # py3
     >>> _make_line_map('box',
     ... StringIO('''// ###sourceLocation(file: "foo.bar", line: 3)