Two build systems

[dimbleby]

the switch to poetry at #63 seems to have been a bit half-finished, and has left this project in a confusing state

in particular, https://pypi.org/project/okta-jwt-verifier/0.2.6/#files has both okta_jwt_verifier-0.2.6-py3-none-any.whl (built by poetry) and okta_jwt_verifier-0.2.6-py2.py3-none-any.whl (built by setuptools), and these have different sets of requirements, it is a mess

the two commits here:

• remove the vestiges of setuptools, if you are going to use poetry then let's use poetry
• removes unnecessary requirements and relaxes unnecessarily tight constraints