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Closes: #83

Add `__override__` attribute to @OverRide

As per discussion in
https://mail.python.org/archives/list/[email protected]/thread/TOIYZ3SNPBJZDBRU3ZSBREXV2NNHF4KW/,
we believe this is a good feature to have (as best-effort: it will not
work in all cases if the user does not understand decorator implementations
enough to get the right order, and it will alwasys fail if a decorator
output does not support dynamic attributes).

We decided to support this for two reasons:
- it was a direct request from the `overrides` library owner
- `typing_extensions.final` has similar behavior

Tests:
```
cd src
python -m unittest test_typing_extensions.py
```

Lints:
```
flake8
flake8 --config=.flake8-tests src/test_typing_extensions.py
```

…94)

Fixes #99

Fixes a typo in 5f9258d. We might have to wait a while before Ubuntu-2004 is released :)

…ped_dict_test_helper.py (#97)

See PEP 702, python/peps#2942.