Use `Self` return type where applicable

[ffl096]

The Self return type has been introduced in Python 3.11. It allows to express that the return type is of the current enclosed class while correctly considering subclasses.

As usual, the Self type is backported to older versions of Python in typing-extensions. We can use the native implementation typing.Self once Python 3.11+ is the minimum version.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.36%. Comparing base (a5684a1) to head (70285de).

:exclamation: Current head 70285de differs from pull request most recent head 34f28dd

Please upload reports for the commit 34f28dd to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
- Coverage   97.36%   97.36%   -0.01%     
==========================================
  Files          37       36       -1     
  Lines        3536     3531       -5     
==========================================
- Hits         3443     3438       -5     
  Misses         93       93              

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