Fix python compiler generated all_structs with type_hints

Currently with type_hints enabled, we do not explicitly declare the type
of generated all_structs. It's meant to be a list of different types,
but that will cause problems, as the generated code is usually like
this:

    all_structs = []
    ...
    all_structs.append(Type1)
    ...
    all_structs.append(Type2)
    ...

The first append infers the type of all_struct to be list[Type1], so
mypyc would start complaining from the 2nd append as the type is
unexpected.

Explicitly declare all_struct to be list[typing.Any] instead, which
fixes the issue.

compiler/cpp/src/thrift/generate/t_py_generator.cc

@@ -437,7 +437,11 @@ void t_py_generator::init_generator() {
            << "from thrift.transport import TTransport" << '\n'
            << import_dynbase_;
 
-  f_types_ << "all_structs = []" << '\n';
+  if (gen_type_hints_) {
+    f_types_ << "all_structs: list[typing.Any] = []" << '\n';
+  } else {
+    f_types_ << "all_structs = []" << '\n';
+  }
 
   f_consts_ <<
     py_autogen_comment() << '\n' <<