enum instance equality broken in pybind11 2.3

[anntzer]

Issue description

In pybind11 2.3, comparing an enum instance with a non-enum attempts to cast the non-enum to int, resulting in broken behaviors when the non-enum is not castable to an int, or a string that can be cast to an int.

Reproducible example code

#include <pybind11/pybind11.h>

enum Foo { a = 1 };

PYBIND11_MODULE(python_example, m) {
    pybind11::enum_<Foo>(m, "Foo")
        .value("a", Foo::a);
}

compile using the python_example example repo.

With pybind11 2.2.4:

$ python -c 'from python_example import Foo; print(Foo.a == None)'
False
$ python -c 'from python_example import Foo; print(Foo.a == "1")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: __eq__(): incompatible function arguments. The following argument types are supported:
    1. (self: python_example.Foo, arg0: python_example.Foo) -> bool
    2. (self: python_example.Foo, arg0: int) -> bool

Invoked with: Foo.a, '1'

With pybind11 2.3:

$ python -c 'from python_example import Foo; print(Foo.a == None)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'
$ python -c 'from python_example import Foo; print(Foo.a == "1")'
True

I would prefer that equality checks always return True or False (consistently with the stdlib's enum.Enum), although I can understand raising a TypeError too. On the other hand, returning True when comparing with the string "1" is clearly incorrect.