missing __builtins__ in the scope with py::exec

[martinRenou]

Issue description

When creating a module using the constructor directly instead of using a macro from pybind11, and when defining multiple functions in it using py::exec, there seem to be an issue. I'm not sure yet if the issue comes from py::exec, but the following code works fine when py::exec is replaced by py::module::import("builtins").attr("exec").

Reproducible example code

py::module my_module("my_module");
std::string my_code = "\n"
       "def foo():\n"
       "    return 4\n"
       "\n"
       "def bar():\n"
       "    return foo()\n";
py::exec(my_code, py::globals(), my_module.attr("__dict__"));

py::print(my_module.attr("foo")()); // Prints "4" as expected
py::print(my_module.attr("bar")()); // Oops, "NameError: global name 'foo' is not defined"

[martinRenou]

According to the Python documentation

Remember that at module level, globals and locals are the same dictionary.

This means that in my case, the code should be:

py::module my_module("my_module");
std::string my_code = "\n"
       "def foo():\n"
       "    return 4\n"
       "\n"
       "def bar():\n"
       "    return foo()\n";
py::exec(my_code, my_module.attr("__dict__"), my_module.attr("__dict__"));

The problem is that it won't be possible to use builtins functions, or even the import keyword. Because the __builtins__ key is not in the my_module.attr("__dict__") dictionary. Which means that this part of the documentation is not implemented in py::exec:

If the globals dictionary does not contain a value for the key builtins, a reference to the dictionary of the built-in module builtins is inserted under that key.