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- TypedDict is provided by the typing module for python >= 3.8
- TypedDict is provided by the typing_extensions module for typing_extensions >=
  3.7.4

but this module is not available as debian buster package.

- mypy does not support condition types defined like that:

```
if CONDITION:
    type = Dict
else:
    type = TypedDict
```

and therefore it's not possible to conditionally declare the type.

but… it does support checks with python version or system platform (cf:
https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks).

Let's import TypedDict from typing when it's available (python >= 3.8), and use
it. Otherwise, let's use a simple dict.

related #9

fix([debian]): python3-logilab-common: debian-changelog-file-contains-invalid-email-address schabot@wilson