Drop support for Python <3.7

[freemansw1]

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Conda-forge is about to drop support for Python 3.7 (https://conda-forge.org/docs/user/announcements.html#dropping-python-3-7) and they have already dropped support for Python 3.6 and below. With that, I think it would be in our best interest to go ahead and require at least Python 3.7 starting with tobac v1.4.0. This will also help us move the code base forward and gives us the option to include features such as type annotations (see #75).

Does anyone see issues with this before I create a PR?

[freemansw1]

Okay, to do this we will need to do the following (as far as I can tell from googling):

• [x] Set the version requirements in setup.py (see, e.g., https://stackoverflow.com/a/45781701/629110 )
• [ ] Set the meta.yml file on our feedstock to require python>=3.7. https://github.com/conda-forge/tobac-feedstock/blob/main/recipe/meta.yaml

[JuliaKukulies]

That sounds like a good plan @freemansw1 !

[freemansw1]

We've now added the deprecation warning on import (#193), so the next step is to update the meta.yml file, which I'll do on release of 1.4.

[freemansw1]

Conda-forge has been updated to require Python 3.7 or greater (https://github.com/conda-forge/tobac-feedstock/pull/18), so this issue is now complete.