[MISC] Migrate typing syntax to Python 3.10+

[sinclert-canonical]

This PR updates the typing signatures to leverage Python 3.10+ syntax.

It is an effort to harmonize typing syntax within the project, as there are some discrepancies regarding how some of it is used throughout the files (i.e. see Optional usages here, and <type> | None usages here).

In short:

- Dict[<type>, <type>]
+ dict[<type>, <type>]

- List[<type>]
+ list[<type>]

- Set[<type>]
+ set[<type>]

- Tuple[<type>]
+ tuple[<type>]

- Optional[<type>]
+ <type> | None

Additional considerations:

• I have not updated the syntax on the postgresql-k8s charm lib, as I thought the Python version requirements may be different for that piece of software.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.55%. Comparing base (68fb053) to head (4ca633f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #816      +/-   ##
==========================================
- Coverage   75.57%   75.55%   -0.03%     
==========================================
  Files          12       12              
  Lines        3108     3105       -3     
  Branches      464      464              
==========================================
- Hits         2349     2346       -3     
  Misses        620      620              
  Partials      139      139              

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[marceloneppel]

Thanks, @sinclert-canonical! LGTM.