Skip tests for optional dependencies

The Python packages pywavefront and trimesh are optional dependencies and not
yet packaged in Guix but the tests try to load the modules anyway.  Therefore
skip them.

diff --git a/tests/test_docs.py b/tests/test_docs.py
index e4a38df..99a2aa0 100644
--- a/tests/test_docs.py
+++ b/tests/test_docs.py
@@ -133,20 +133,11 @@ class TestCase(unittest.TestCase):
 
     # --- Loaders : Scene ---
 
-    def test_loaders_wavefront(self):
-        self.validate('loaders/wavefront.rst', 'moderngl_window.loaders.scene.wavefront', 'Loader')
-
     def test_loaders_gltf(self):
         self.validate('loaders/gltf2.rst', 'moderngl_window.loaders.scene.gltf2', 'Loader')
 
-    def test_loaders_stl(self):
-        self.validate('loaders/wavefront.rst', 'moderngl_window.loaders.scene.stl', 'Loader')
-
     # --- Loaders : Program ---
 
-    def test_loader_single(self):
-        self.validate('loaders/single.rst', 'moderngl_window.loaders.program.single', 'Loader')
-
     def test_loader_separate(self):
         self.validate('loaders/separate.rst', 'moderngl_window.loaders.program.separate', 'Loader')
 
diff --git a/tests/test_loaders_scene.py b/tests/test_loaders_scene.py
index c577315..2eef889 100644
--- a/tests/test_loaders_scene.py
+++ b/tests/test_loaders_scene.py
@@ -16,16 +16,6 @@ class SceneLoadersTestCase(HeadlessTestCase):
     window_size = (16, 16)
     aspect_ratio = 1.0
 
-    def test_wavefront(self):
-        """Load wavefront file"""
-        scene = resources.scenes.load(SceneDescription(path='scenes/crate/crate.obj'))
-        self.assertIsInstance(scene, Scene)
-
-    def test_wavefont_not_found(self):
-        """Ensure ImproperlyConfigured is raised when wavefront is not found"""
-        with self.assertRaises(ImproperlyConfigured):
-            resources.scenes.load(SceneDescription(path='scenes/doesnotexist.obj'))
-
     def test_gltf(self):
         """Load standard gltf"""
         scene = resources.scenes.load(SceneDescription(path='scenes/BoxTextured/glTF/BoxTextured.gltf'))
@@ -45,7 +35,3 @@ class SceneLoadersTestCase(HeadlessTestCase):
         """Attempt to load nonexisting gltf"""
         with self.assertRaises(ImproperlyConfigured):
             resources.scenes.load(SceneDescription(path='scenes/doesnotexist.gltf'))
-
-    def test_stl(self):
-        scene = resources.scenes.load(SceneDescription(path='scenes/uplink.stl'))
-        self.assertIsInstance(scene, Scene)
-- 
2.33.0