BlendPath: don't use os.path.join() for slash notations

The BlendPath should just use forward slashes, and not be depending on the
current platform.

blender_asset_tracer/bpathlib.py

@@ -79,13 +79,13 @@ class BlendPath(bytes):
         sub = BlendPath(subpath)
         if sub.is_absolute():
             raise ValueError("'a / b' only works when 'b' is a relative path")
-        return BlendPath(os.path.join(self, sub))
+        return BlendPath(self.rstrip(b'/') + b'/' + sub)
 
     def __rtruediv__(self, parentpath: bytes):
         """Slash notation like pathlib.Path."""
         if self.is_absolute():
             raise ValueError("'a / b' only works when 'b' is a relative path")
-        return BlendPath(os.path.join(parentpath, self))
+        return BlendPath(parentpath.rstrip(b'/') + b'/' + self)
 
     def to_path(self) -> pathlib.PurePath:
         """Convert this path to a pathlib.PurePath.

tests/test_bpathlib.py

@@ -45,6 +45,11 @@ class BlendPathTest(unittest.TestCase):
         with self.assertRaises(ValueError):
             b'/root/and' / BlendPath(b'/parent.blend')
 
+        # On Windows+Python 3.5.4 this resulted in b'//root//parent.blend',
+        # but only if the root is a single term (so not b'//root/and/').
+        self.assertEqual(BlendPath(b'//root/parent.blend'),
+                         BlendPath(b'//root/') / b'parent.blend')
+
     def test_mkrelative(self):
         self.assertEqual(b'//asset.png', BlendPath.mkrelative(
             Path('/path/to/asset.png'),