available_frontends: Import pytest locally to make dependency optional

loki/frontend/util.py

@@ -10,15 +10,14 @@
 import codecs
 from codetiming import Timer
 from more_itertools import split_after
-import pytest
 
 from loki.ir import (
     NestedTransformer, FindNodes, PatternFinder, Transformer,
     Assignment, Comment, CommentBlock, VariableDeclaration,
     ProcedureDeclaration, Loop, Intrinsic, Pragma
 )
 from loki.frontend.source import join_source_list
-from loki.logging import warning, perf
+from loki.logging import warning, perf, error
 from loki.tools import group_by_class, replace_windowed, as_tuple
 
 
@@ -92,6 +91,12 @@ def my_test(frontend):
     else:
         skip = {}
 
+    try:
+        import pytest  # pylint: disable=import-outside-toplevel
+    except ImportError as e:
+        error('Pytest is not installed.')
+        raise e
+
     from loki import frontend  # pylint: disable=import-outside-toplevel,cyclic-import
 
     # Unavailable frontends