Change requirements.txt for compatible releases

Currently the `requirements.txt` requires exact `major.minor.patch` versions, and Dependabot opens PRs to upgrade even patch releases.

Because we are building documentation with these dependencies, and not running a real-time/production system, we should change the `requirements.txt` to use the [compatible releases](https://packaging.python.org/en/latest/specifications/version-specifiers/#compatible-release) syntax on just `major.minor` version. Patch releases aren't as important for us to upgrade to, as these dependencies are stable and creating working docs for the project.