fix: use build package for build process

[Justintime50]

Description

Installing/building via setup.py directly is deprecated. We are migrating to the build package to do the same thing for us. I inspected the output package and everything is the same as before (sdist and wheels are built as before with a simpler interface and include all the same items as before. I've had great success with this package on dozens of personal projects over the last year.

Testing

Ran make build and got the output of Successfully built easypost-7.9.0.tar.gz and easypost-7.9.0-py3-none-any.whl, the added benefit of build is it creates a temporary virtual env just for this build which should isolate this flow even better.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)