Separate runtime and dev dependencies

[laxmena]

Summary

• Moves pytest out of requirements.txt into a new requirements-dev.txt
• Removes typing from requirements (built-in since Python 3.7)
• Updates GitHub Actions and CircleCI to install requirements-dev.txt for test runs
• Production installs (pip install -r requirements.txt) no longer pull in pytest

Closes #4

Test plan

• Verify CI passes with updated workflow files
• Verify requirements.txt only contains requests
• Verify requirements-dev.txt includes -r requirements.txt so it still installs runtime deps