[update] Improve package setup

[jon-tow]

This PR addresses package setup issues with the following updates:

• Adds package finding to setup.cfg to avoid "multiple top-level packages" errors during new environment installs.
  • Related issue: Installation error due to multiple top-level packages  #41
• Updates README.md installation instructions to use pip install -e ..., as python setup.py ... is deprecated and should be replaced with pip tools (reference guide).
• Adds separate dev dependencies to setup.cfg to avoid forcing dev dependencies on package install.
  • Adopts pytest as the test runner.
• Renames unittests to tests to avoid confusion with unittest module and adhere to common Python unit-test directory naming.

[Dahoas]

This is great thanks!