Implement Canonical URLs

[tristanlatr]

Fixes #608

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.22%. Comparing base (68cb9f8) to head (8b33a8a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #821      +/-   ##
==========================================
+ Coverage   92.20%   92.22%   +0.01%     
==========================================
  Files          47       47              
  Lines        8495     8513      +18     
  Branches     1882     1886       +4     
==========================================
+ Hits         7833     7851      +18     
  Misses        385      385              
  Partials      277      277              

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[github-actions]

Diff from pydoctor_primer, showing the effect of this PR on open source code:

numpy (https://github.com/numpy/numpy): typechecking got 1.08x slower (148.7s -> 160.9s)
(Performance measurements are based on a single noisy sample)

[tristanlatr]

This is ready to be merged!