Fix typeguard import hook location.

Package ``typeguard`` moved the function ``install_import_hook`` to
the top level module for version 3+. This adapts the test runner
to accept both the old and the new location.

runtests.py

@@ -5,7 +5,13 @@
 if bool(os.environ.get('NUMBA_USE_TYPEGUARD')):
     # The typeguard import hook must be installed prior to importing numba.
     # Therefore, this cannot be part of the numba package.
-    from typeguard.importhook import install_import_hook
+    try:
+        # version 3+ exports this at the top level
+        from typeguard import install_import_hook
+    except ImportError:
+        # try location for version 2.x
+        from typeguard.importhook import install_import_hook
+
     install_import_hook(packages=['numba'])
 
 # ensure full tracebacks are available and no help messages appear in test mode