fix: install typing-extensions when using Python > 3.8. Closes #540

[Oskar-Bonde]

This is a fix for the following bug. typing-extensions is now installed when using Python > 3.8

Describe the bug
typing extensions is not a required dependency when using Python > 3.8. So when using Python > 3.8 an import error is raised.

To Reproduce
Install dataclasses-avroschema using Python > 3.8. Make sure no other package is installing typing-extensions. Import error will be raised when trying to use AvroModel because of the following import: from typing_extensions import get_args.

Expected behavior
No import error.

Fix by updating pyproject.toml, remove python version requirement from typing-extensions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1d6ef42) 99.18% compared to head (a7fd2ae) 99.18%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #541   +/-   ##
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  Coverage   99.18%   99.18%           
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  Files          30       30           
  Lines        1712     1712           
  Branches      307      307           
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  Hits         1698     1698           
  Misses          6        6           
  Partials        8        8           

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