Issue title: Importing tensorflow causes a crash on exit
 Issue body: ## 🐛 Bug Reports

### 🌍 Environment

 - Your operating system and version: Ubuntu 18.04.4 LTS (Bionic Beaver)
 - Your python version: Python 3.7.8
 - How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: Installed via apt. No virtualenv.
 - Your Rust version (`rustc --version`): 1.45.0
 - Your PyO3 version: 0.11.1
 - Have you tried using latest PyO3 master (replace `version = "0.x.y"` with `git = "https://github.com/PyO3/pyo3")?`: Yes

### 💥 Reproducing

Tensorflow version: 2.1.0, installed via pip: `pip3 install tensorflow==2.1.0`

Minimal reproduction:

```rust
use pyo38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5prelu38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5*;

fn main() {
    let gil = Python38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5quire_gil();
    let py = gil.python();

    let _mod = PyModul38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5import(py, "tensorflow").unwrap();
}
```
On exit:
```
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
          Running `target/debug/pyo3`
corrupted double-linked list
[1]    4387 abort      cargo run
```
or

```
$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
          Running `target/debug/pyo3`
corrupted size vs. prev_size
[1]    5299 abort      cargo run
```

Running import tensorflow directly in python (and exiting the interpreter) exits normally.

Looks like a similar issue to [Importing mxnet causes a segfault on exit #1044](https://github.com/PyO3/pyo3/issues/1044#issue-657897568).

If I comment out `li38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5atexit(finalize)` as @kngwyu suggested in [comment](https://github.com/PyO3/pyo3/issues/1044#issuecomment-660464673), the error does not happen and the process exits successfully.

 Comments: 
 Comment 0: #1355 has now merged - we decided to disable finalizing by default because as we see it can lead to weird crashes like this. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'PyO3/pyo3', 'issue_id': 668750401, 'repo_stars': 6736.0}