Add support for type alias statements

[tristanlatr]

Type alias statements were introduced in Python 3.12

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.69%. Comparing base (fe29bb7) to head (3a3a7ae).

Files	Patch %	Lines
pydoctor/astbuilder.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #771   +/-   ##
=======================================
  Coverage   92.69%   92.69%           
=======================================
  Files          47       47           
  Lines        8337     8354   +17     
  Branches     1846     1849    +3     
=======================================
+ Hits         7728     7744   +16     
  Misses        349      349           
- Partials      260      261    +1     

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[tristanlatr]

TODO next: Add support for type parameters in typer aliases, functions and classes.

[github-actions]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅

[tristanlatr]

Replaced by #772