tests: add Filename unit tests for fspath protocol in particular

tests/dtoolutil/test_filename.py

@@ -0,0 +1,24 @@
+from panda3d.core import Filename
+import sys, os
+import pytest
+
+
+@pytest.mark.skipif(sys.version_info < (3, 6), reason="Requires Python 3.6")
+def test_filename_fspath():
+    fn = Filename.from_os_specific(__file__)
+    assert os.fspath(fn) == fn.to_os_specific_w()
+
+
+@pytest.mark.skipif(sys.version_info < (3, 6), reason="Requires Python 3.6")
+def test_filename_open():
+    fn = Filename.from_os_specific(__file__)
+    open(fn, 'rb')
+
+
+@pytest.mark.skipif(sys.version_info < (3, 4), reason="Requires Python 3.4")
+def test_filename_ctor_pathlib():
+    pathlib = pytest.importorskip('pathlib')
+
+    path = pathlib.Path(__file__)
+    fn = Filename(path)
+    assert fn.to_os_specific_w() == str(path)