Pull requests into main branch should check for semver bump

[zomglings]

The logic is simple - we should:

1. pip install locust from PyPI
2. get the version of the PyPI locust
3. compare it to the version of the locust in the PR
4. if the locust version from the PR is not semver greater than the version of locust from PyPI, fail the check

We should add a console_scripts entry for locust.version in setup.py.