Make this project source installable via Pip

[dbanty]

Is your feature request related to a problem? Please describe.
Originally requested by @TMiguelT in #119. The goal is to be able to pip install -U git+https://github.com/triaxtec/openapi-python-client.git.

Describe the solution you'd like
Auto-generate a setup.py from pyproject.toml using GitHub Actions so it's kept up to date.

Additional context
This will probably also make this installable via pipx from source.

[multimeric]

As I said elsewhere, I think with pip 19.0 onwards, this should just work automatically, as long as you define the [build-system] section correctly: https://pip.pypa.io/en/stable/reference/pip/#pep-517-and-518-support. Because then pip understands that it needs poetry in order to build the package.

[dbanty]

Oh interesting, I knew it could read some data from pyproject.toml but didn't realize you could tell it to require packages. I'll give this a shot for sure.

[dbanty]

Awesome, it works! Thanks @TMiguelT, I'll probably do something similar for #120 in the short term too.