Python gen for array within struct not set to staticmethod for length/isnone

[dshekter]

The ...Length and ...IsNone functions are constant for an array within a struct (which is kind of the point I think?) so the functions should be static in the py and pyi. These are located:

flatbuffers/src/idl_gen_python.cpp

Lines 481 to 482 in bab1075

	stub << " def " << name << "Length(self) -> int: ...\n";
	stub << " def " << name << "IsNone(self) -> bool: ...\n";

flatbuffers/src/idl_gen_python.cpp

Line 804 in bab1075

void GetVectorLen(const StructDef& struct_def, const FieldDef& field,

and

flatbuffers/src/idl_gen_python.cpp

Line 825 in bab1075

void GetVectorIsNone(const StructDef& struct_def, const FieldDef& field,