Fix last remaining PyMC3 occurrences & broken link

[Armavica]

What is this PR about?
This PR fixes the last occurrences of PyMC3 in the repo, and a broken README link.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• ...

Bugfixes / New features

• ...

Docs / Maintenance

• Fix the last occurrences of PyMC3 in the repo, and a broken README link.

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[codecov]

Codecov Report

Merging #6133 (729934b) into main (4ea8dde) will increase coverage by 24.36%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6133       +/-   ##
===========================================
+ Coverage   68.54%   92.90%   +24.36%     
===========================================
  Files         101      101               
  Lines       20941    20941               
===========================================
+ Hits        14353    19455     +5102     
+ Misses       6588     1486     -5102     

Impacted Files	Coverage Δ
pymc/tests/distributions/test_continuous.py	99.76% <ø> (ø)
pymc/sampling.py	82.13% <0.00%> (+0.11%)	⬆️
pymc/distributions/multivariate.py	92.31% <0.00%> (+0.12%)	⬆️
pymc/step_methods/metropolis.py	83.40% <0.00%> (+0.41%)	⬆️
pymc/model.py	88.23% <0.00%> (+0.65%)	⬆️
pymc/distributions/distribution.py	95.87% <0.00%> (+0.91%)	⬆️
pymc/backends/arviz.py	90.61% <0.00%> (+4.48%)	⬆️
pymc/smc/sample_smc.py	83.22% <0.00%> (+5.36%)	⬆️
pymc/util.py	77.64% <0.00%> (+5.88%)	⬆️
pymc/backends/ndarray.py	79.46% <0.00%> (+6.25%)	⬆️
... and 31 more