python3Packages.pymeshlab: add missing dep

fix:

```
Executing pythonImportsCheckPhase
Check whether the following modules can be imported: pymeshlab
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; import importlib; list(map(lambda mod: importlib.import_module(mod), sys.argv[1:]))
                                  ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 1, in <lambda>
    import sys; import importlib; list(map(lambda mod: importlib.import_module(mod), sys.argv[1:]))
                                                       ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/nix/store/60m4rxhg2fldqaak400c0lry96ijrzqn-python3-3.13.13/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1395, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1023, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/nix/store/jmjl11ib1a84chnzqrajixraa5v1f0sh-python3.13-pymeshlab-2025.7.post1/lib/python3.13/site-packages/pymeshlab/__init__.py", line 11, in <module>
    from .pmeshlab import *
ImportError: libSM.so.6: cannot open shared object file: No such file or directory
```

While here add strictDeps & __structuredAttrs

pkgs/development/python-modules/pymeshlab/default.nix

@@ -17,6 +17,7 @@
 
   # buildInputs
   libsForQt5,
+  libsm,
   llvmPackages,
   glew,
   vcg,
@@ -60,6 +61,7 @@ buildPythonPackage {
   buildInputs = [
     glew
     libsForQt5.qtbase
+    libsm
     vcg
   ]
   ++ lib.optionals stdenv.cc.isClang [
@@ -96,6 +98,9 @@ buildPythonPackage {
 
   pythonImportsCheck = [ "pymeshlab" ];
 
+  strictDeps = true;
+  __structuredAttrs = true;
+
   meta = {
     description = "Open source mesh processing python library";
     homepage = "https://github.com/cnr-isti-vclab/PyMeshLab";