Make imports compatible with PyTensor/PyMC v5

[michaelosthege]

I changed the aesara imports to try PyTensor first, and fall back to aesara otherwise.

Mentions of Aesara were changed to PyTensor, and in pymc.py I had to change where softmax comes from because aesara.tensor.nnet was removed.

Closes #611

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[michaelosthege]

cc @tomicapretto @aloctavodia

[canyon289]

IMO we should just hard cut over. Theres no reason to stay on Aesara and it just means that we may get user bug reports from two backends rather than one.

[michaelosthege]

I would recommend to keep compatibility for at least one release and print a warning.

In the four packages I patched in the past few days I maintained compatibility with v4 because it simplifies updating my environments and Docker images.

In any case please take over from here - I'm not even using bambi myself 🙊

[canyon289]

Ill merge it. Hopefully no one gets angry at me

[codecov-commenter]

Codecov Report

Merging #613 (548e1ee) into main (c256549) will decrease coverage by 0.20%.
The diff coverage is 69.44%.

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
- Coverage   82.21%   82.00%   -0.21%     
==========================================
  Files          40       40              
  Lines        3093     3107      +14     
==========================================
+ Hits         2543     2548       +5     
- Misses        550      559       +9     

Impacted Files	Coverage Δ
bambi/families/link.py	65.71% <ø> (ø)
bambi/backend/utils.py	76.92% <50.00%> (-13.08%)	⬇️
bambi/backend/pymc.py	81.42% <55.55%> (-0.84%)	⬇️
bambi/backend/terms.py	93.87% <60.00%> (-1.27%)	⬇️
bambi/backend/links.py	89.65% <80.00%> (-6.50%)	⬇️
bambi/families/multivariate.py	95.38% <100.00%> (ø)

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