Remove get_refcnt from public api

[lfn3]

This is an attempt to fix #3357.
The function is used relatively frequently in tests, so have left it with #[cfg(test)] and pub(crate).
This avoids coating the tests with unsafe, and something that doesn't indicate intent as well.
(As has been done in test_inheiritance.rs)

TODO:

• Fix docs that refer to get_refcnt as an example - is there another function I can use instead?

[LilyFoote]

Looks like a good start to me.

I had a look at the failing tests and found these places we'll need more changes:

• We'll need a different example (maybe is_truthy?):

  pyo3/guide/src/class.md

  Lines 855 to 859 in d2dca21

  	// Take a GIL-indepedent reference when you want to store the reference elsewhere.
  	#[pyfunction]
  	fn print_refcnt(my_class: Py<MyClass>, py: Python<'_>) {
  	println!("{}", my_class.get_refcnt(py));
  	}

• We'll need a different example in

  pyo3/guide/src/migration.md

  Lines 1537 to 1558 in d2dca21

  	### All `PyObject` and `Py<T>` methods now take `Python` as an argument
  	<details>
  	<summary><small>Click to expand</small></summary>
  	Previously, a few methods such as `Object::get_refcnt` did not take `Python` as an argument (to
  	ensure that the Python GIL was held by the current thread). Technically, this was not sound.
  	To migrate, just pass a `py` argument to any calls to these methods.
  	Before:
  	```rust,compile_fail
  	# pyo3::Python::with_gil(|py| {
  	py.None().get_refcnt();
  	# })
  	```
  	After:
  	```rust
  	# pyo3::Python::with_gil(|py| {
  	py.None().get_refcnt(py);
  	# })
  	```
  	</details>

  or to mark it as compile_fail too. @davidhewitt do you have a preference here?

[LilyFoote]

What was the reasoning for changing these? If I understand the issue, we want to keep get_refcnt for internal tests like this.

[Dr-Emann]

The tests in tests are integration tests, they are compiled as a separate crate, and thus don't have access to private functions internal to the crate.

[LilyFoote]

Ah! That makes sense!