[PyROOT] Fix a warning regarding a const return type.

Fix a warning when ACLiC runs:
  /Users/sftnight/ROOT-CI/build/bindings/pyroot/cppyy/cppyy/test/./stltypes.h:81:12: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
     81 |     static const size_t size() { return sz; }
        |            ^~~~~

Author: hageboeck

bindings/pyroot/cppyy/cppyy/test/stltypes.h

@@ -78,7 +78,7 @@ class stl_like_class2 {
     value_type fData[sz];
 
 public:
-    static const size_t size() { return sz; }
+    static size_t size() { return sz; }
     value_type& operator[](ptrdiff_t i) { return fData[i]; }
 };