Theano dependency is pinned in 4 places

[twiecki]

A bit unfortunate that we specify the exact version in 4 places now. Do we even need it for the -dev environments? My understanding is that these are just the dependencies in addition to what is in requirements.txt.

[MarcoGorelli]

In the future, will PyMC release have a minimum theano-pymc/aesara version, or will each PyMC release have one particular theano-pymc release associated with it?

If the latter, then it should probably only be defined in a single place (or there should be checks that the different places it's defined match up). If it's the latter, then in the other three places it's pinned (conda-envs/environment-dev-py36.yml, conda-envs/environment-dev-py37.yml, conda-envs/environment-dev-py38.yml), the pins could be mixed up a bit between them so that different CI jobs run tests with different versions.

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I presume the requirements-dev.txt is for people (like @rpgoldman ?) who aren't developing in a conda environment?

In pandas there's a script to generate pip requirements from conda: https://github.com/pandas-dev/pandas/blob/master/scripts/generate_pip_deps_from_conda.py , if you want we could add that to make sure they stay consistent.

[twiecki]

@MarcoGorelli for the time being we will probably be pretty interlocked with specific theano versions.

I like the idea with generating the pip files from the conda env.