Simplify build process using poetry v2

I tried installing this library on Big Sur, and I could not solve errors with finding libraries installed via Homebrew. However, I tried @bsdz's fork and it installed great!

Since #34 is closed due to a few needed changes, I've tried to apply the fixes here in hopes that we can merge this PR. Those are:

Pinned python version and dependency versions in pyproject.toml to *

This forces the way-out-of date numpy 1.16 (newest 1.20) and pillow 5.4 (newest 8.1) to be installed (see poetry.lock - this is to keep compatibility with old pythons). These build and test correctly on my linux machine, but on mac I have to manually install the latest pillow before I poetry build because old pillow doesn't compile.\ This is just for running poetry commands within the repo for developing. pip installing the project works fine and uses the latest versions.

Alternatively, if python is limited to >=3.6 (imo python2 should die already and python 3.6 is plenty old, but your call), poetry uses the latest versions of numpy and pillow.
Replaced black with nose in dependencies (as it didn't look like black was used, and nose is needed for tests)
Formatted to make flake8 happy
I haven't squashed everything so I can separate my few changes from @bsdz's hard work. If GitHub doesn't allow squashing while merging, I'm happy to squash them after the code is reviewed.

To test: poetry install && poetry build To just install: pip install git+https://github.com/rianadon/pypotrace.git#master

flupke / pypotrace

Simplify build process using poetry v2 #35