[issue40929] Multiprocessing. Instance of the custom class is missing in subprocesses

[cyberlis]

New submission from cyberlis <cyberlis at rccraft.ru>:

When I use python3.7 everything works correctly and every subprocess has its own `Hello` instance. When I use python3.8 subprocesses do not have an instance of `Hello` class.
Is this behavior correct? 

My code to reproduce an issue

```python
from concurrent.futures.process import ProcessPoolExecutor
from concurrent.futures import as_completed
class Hello:
    _instance = None
    def __init__(self):
        print("Creating instance of Hello ", self)
        Hello._instance = self
def worker():
    return Hello._instance
def main():
    hello = Hello()
    with ProcessPoolExecutor(max_workers=2) as pool:
        futures = []
        for _ in range(4):
            futures.append(pool.submit(worker))

        for future in as_completed(futures):
            print(future.result())
if __name__ == "__main__":
    main()
```

Output
```bash
pyenv local 3.7.6
python main.py

Creating instance of Hello  <__main__.Hello object at 0x102f48310>
<__main__.Hello object at 0x103587410>
<__main__.Hello object at 0x1035874d0>
<__main__.Hello object at 0x103587110>
<__main__.Hello object at 0x1035871d0>

pyenv local 3.8.1
python main.py

Creating instance of Hello  <__main__.Hello object at 0x102104d90>
None
None
None
None
```

----------
files: main.py
messages: 371117
nosy: cyberlis
priority: normal
severity: normal
status: open
title: Multiprocessing. Instance of the custom class is missing in subprocesses
type: behavior
versions: Python 3.8
Added file: https://bugs.python.org/file49223/main.py

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40929>
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