Add __builtins__ to globals argument of py::exec and py::eval if not present

[YannickJadoul]

Description

Python (even 2.7!) adds __builtins__ when calling exec or eval:

Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> d = {}
>>> d.keys()
dict_keys([])
>>> exec("print(42)", d)
42
>>> d.keys()
dict_keys(['__builtins__'])

Python 2.7.17 (default, Sep 30 2020, 13:38:04) 
[GCC 7.5.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> d = {}
>>> exec("print 42", d)
42
>>> d.keys()
['__builtins__']

For the C API's PyRun_String, this only happens since Python 3.8 (python/cpython#13362; see #1091 (comment)). It would make sense to me to backport/downport/sideport/port this to pybind11 on all versions of Python, for the sake of consistency. No?

Closes #1091
Closes #2557
Closes #1654 (thanks, @bstaletic!)

Suggested changelog entry:

`py::exec`, `py::eval`, and `py::eval_file` now add the builtins module as ``"__builtins__"`` to their ``globals`` argument, better matching `exec` and `eval` in pure Python.

[bstaletic]

Fixes #1654 as well.

[martinRenou]

Thanks a lot @YannickJadoul

[wjakob]

Looks great!

[rwgk]

This looks good to me as-is, but what do you think about moving the new code into inline void detail::ensure_global_contains_builtins()? That way you'd only need one #if (inside that function).

[YannickJadoul]

what do you think about moving the new code into inline void detail::ensure_global_contains_builtins()

I'm not entirely convinced on which side to pick here. On a high level, definitely, avoid code duplication. In practice ... I'm a bit in doubt whether it looks better. But I guess it also doesn't look worse.

[YannickJadoul]

One or two reviews of this last commit, please?

[YannickJadoul]

Thanks, all!