Fix enum value's __int__ returning non-int when underlying type is bool or of char type (#1334)

* Use equivalent_integer for enum's Scalar decision

* Add test for char underlying enum

* Support translating bool type in enum's Scalar

* Add test for bool underlying enum

* Fix comment in test

* Switch from `PYBIND11_CPP20` macro to `PYBIND11_HAS_U8STRING`

* Refine tests

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>