Fix type hints for Python 3.8

[bouweandela]

Description

This fixes the issue with type hints breaking the program in Python 3.8.

It removes the need for allowing implicit optional function arguments.

It also updates the mypy version to a version that supports the --implicit-optional argument. I couldn't find in the changelog when exactly this feature was introduced, so I just pinned it to the latest version or newer. Maybe this is no longer needed, since I removed the implicit_optional command line argument in this pull request? It might be nice to have some lower version though.

Fixes the type hints part of #1793.
Fixes #1789

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[codecov]

Codecov Report

Merging #1795 (b0a5d63) into main (ef33a2d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1795   +/-   ##
=======================================
  Coverage   91.50%   91.51%           
=======================================
  Files         202      202           
  Lines       10904    10908    +4     
=======================================
+ Hits         9978     9982    +4     
  Misses        926      926           

Impacted Files	Coverage Δ
esmvalcore/_task.py	71.62% <100.00%> (+0.06%)	⬆️
esmvalcore/cmor/table.py	94.50% <100.00%> (+0.01%)	⬆️
esmvalcore/config/_config_validators.py	92.75% <100.00%> (+0.05%)	⬆️
esmvalcore/config/_logging.py	97.67% <100.00%> (ø)
esmvalcore/experimental/recipe.py	90.32% <100.00%> (ø)
esmvalcore/experimental/recipe_metadata.py	93.24% <100.00%> (+0.09%)	⬆️
esmvalcore/experimental/recipe_output.py	82.45% <100.00%> (ø)
esmvalcore/experimental/utils.py	56.25% <100.00%> (ø)

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[zklaus]

LGTM. Do we need a strategy or issue or something to remind us to remove the __future__ imports when we arrive in the future, i.e. when we drop 3.8?

[bouweandela]

Thanks for the quick review @zklaus! @ESMValGroup/technical-lead-development-team Would someone have time to merge this?

Do we need a strategy or issue or something to remind us to remove the future imports when we arrive in the future, i.e. when we drop 3.8?

I'm not sure, maybe it will generate more noise than be helpful to create issues for these type of things.

[schlunma]

Thanks guys!