Drop Python 3.7

[ferrine]

Description

Due to pymc supports Python 3.8+ it makes no sense to keep Python 3.7 here

https://github.com/pymc-devs/pymc/blob/0de47bcdc674f085acc91dc341f4d74c74078d13/setup.py#L74

[ricardoV94]

The reason to keeping support is that Google Colab keeps lagging behind -.-

[ferrine]

How could it happen if pymc is already on 3.8?

[twiecki]

We made a special 3.7 release for colab.

[ricardoV94]

Colab is now on 3.8, we can drop and maybe start testing on 3.11 :)

[ricardoV94]

Closed via #210