Fixed type annotations for Python 3.8

[schlunma]

Description

#1694 added type annotations that are only available since Python 3.9; thus, tests are failing with Python 3.8. This PR fixes this.

Checklist

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[codecov]

Codecov Report

Merging #1700 (c7c3301) into main (613f08c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1700   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files         205      205           
  Lines       11191    11192    +1     
=======================================
+ Hits        10241    10242    +1     
  Misses        950      950           

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/native_datasets.py	100.00% <100.00%> (ø)

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[schlunma]

Tests on 3.8 now run successful: https://github.com/ESMValGroup/ESMValCore/runs/7874314356?check_suite_focus=true

[remi-kazeroni]

Looks good to me, thanks