Drop Python 3.7 support and upgrade dependencies

[btjanaka]

Description

Python 3.7 is now end of life; thus v0.6.0 will drop support for it. Note that most dependencies can remain the same since the versions we depend on support Python 3.7 and up, which includes Python 3.8.

TODO

• [x] Upgrade all CI to use at least python 3.8
• [x] Update tags in setup.py
• [x] Update documentation to only start at python 3.8
• [x] Upgrade scikit learn to 1.1.0 as we had a breaking change that we had to account for in cvt archive. This upgrade is feasible since scikit learn 1.1.0 requires Python 3.8+. This upgrade also allows us to remove semantic version as a dependency.
• [x] Upgrade pinned reqs for visualize test to closely match the versions we develop with. This also necessitates updating the baseline images.

Website

• [x] Update website to mention supporting python 3.8 and up

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go