Add PEP621 format toml file

Add pep621_pyproject.toml

* This TOML file complies with PEP517 & PEP518 & PEP621

This link is for the PYPI package generated by this TOML file.

* https://pypi.org/project/locust-je-test-pep621/2.15.2.dev8/

I don't want to overwrite old pyproject.toml so i call this file pep621_pyproject.toml

Remove duplicate zip_safe

Pull #2337 and add pep621 to pyproject.toml

Remove duplicate setuptools_scm on build system requires & old pyproject

from license = { file = "LICENSE"}
to license = { text = "MIT"}

because we have this options
license-files = ["LICENSE"]

Delete setup.cfg & setup.py

Replace 'setup.cfg' with 'pyproject.toml'

…ist/* && python3 -m build .

Replace
rm -f dist/* && python3 setup.py sdist bdist_wheel
to
 rm -f dist/* && python3 -m build .

Nuke examples\vagrant\vagrant.sh and
add pip upgrade build on Makefile