Narrow AST parsing result type to ast.Module at all times

[bswck]

According to this overload of ast.parse for Python versions less than 3.13 and this overload of ast.parse for Python versions greater or equal to 3.13, parse_stub_file can only return ast.Module because the default value of the mode param in ast.parse is "exec" and parse_stub_file doesn't explicitly specify the argument for mode. Consequently, get_stub_ast can only return ast.Module or None.