Enable type hint checking

[bouweandela]

🚀 Pull Request

Description

Enable mypy checks and fix issues with current type hints.

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• benchmark_this Request that this pull request be benchmarked to check if it introduces performance shifts

[codecov]

Codecov Report

Attention: Patch coverage is 98.52941% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.77%. Comparing base (1bea71c) to head (74d32b9).

Files	Patch %	Lines
lib/iris/analysis/maths.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5956      +/-   ##
==========================================
+ Coverage   89.76%   89.77%   +0.01%     
==========================================
  Files          90       90              
  Lines       22969    22984      +15     
  Branches     5029     5031       +2     
==========================================
+ Hits        20618    20634      +16     
+ Misses       1620     1619       -1     
  Partials      731      731              

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[trexfeathers]

Massive ❤ for tackling this @bouweandela.

I have some comments for you, and I don't know if @stephenworsley might be half-way through a review?