Cannot import name 'solve_triangular' from 'pytensor.tensor.linalg'

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The PyMC team took over development of Theano a while back, after it was sunset by the original developers. The package called “theano” is just for archive purposes at this point, for use with legacy code. The modern descendant of Theano that is currently maintained was renamed to “pytensor”, and that’s all you need to power PyMC now.