hghave: support HGMODULEPOLICY for pure

HGMODULEPOLICY was introduced in 4374d819ccd5

tests/hghave.py

@@ -460,7 +460,10 b' def has_py3k():'
 
 @check("pure", "running with pure Python code")
 def has_pure():
-    return os.environ.get("HGTEST_RUN_TESTS_PURE") == "--pure"
+    return any([
+        os.environ.get("HGMODULEPOLICY") == "py",
+        os.environ.get("HGTEST_RUN_TESTS_PURE") == "--pure",
+    ])
 
 @check("slow", "allow slow tests")
 def has_slow():