Poncho pinned

[btovar]

Proposed Changes

This pr adds “pinned” to the poncho spec. The target use case is something like topcoffea, where we want to create an environment compatible with another environment. For example, we want numpy in the former, which means it should be the same version of numpy in the latter. The “pinned” spec allows specifying versions of packages that should be used in case they need to be installed as a dependency.

As an example:

{
    "conda": {
        "channels": ["conda-forge"],
        "packages": [
            "ndcctools",
        ]
    },
    "pinned": {
      "conda": {
        "python": "3.8.20=h4a871b0_2_cpython"
      }
    }
}

The above will try to install ndcctools, which depends on python. Python is pinned to version 3.8.20 and thus the version installed for ndcctools is 7.12.0, the last version that exists with python 3.8 support.

Currently this only works for conda packages. I'm thinking how to do this easily for pip.

If this looks good, I can update the docs.

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