Move project metadata to pyproject.toml, modify build scripts

[mwtoews]

This PR moves the project metadata to pyproject.toml (PEP 621), as supported by setuptools>61, and specifies "setuptools.build_meta" as the build-backend (PEP 517).

For now, keep setup.py, as some components expect this file. Also, the project name is repeated to allow GitHub dependency tracker to work.

It is mostly a 1:1 conversion, with a few exceptions:

• Short description changed to the shorter version found on GitHub
• License identifier is modified to the short identifier used by SPDX
• A content type is specified for the long description (before this failed twine check --strict dist/*)
• Platform is removed, since this is mostly useless metadata not used by pip
• No need to ever specify "Python <4"

Full diff of the METADATA

4,8c4,7
< Summary: Conversion library for common geospatial data formats (GeoJSON/WKT/EWKT/WKB/EWKB/GeoPackage/EsriJson)
< Home-page: https://github.com/geomet/geomet
< Maintainer: Lars Butler
< Maintainer-email: lars.butler@gmail.com
< License: Apache 2.0
---
> Summary: Pure Python conversion library for common geospatial data formats
> Maintainer-email: Lars Butler <lars.butler@gmail.com>
> License: Apache-2.0
> Project-URL: Repository, https://github.com/geomet/geomet
10d8
< Platform: any
22c20,21
< Requires-Python: >=3.7, <4
---
> Requires-Python: >=3.7
> Description-Content-Type: text/x-rst
27d25
< 

Also, there are several changes to the build scripts:

• Change commands that directly invoke setup.py, see Why you shouldn't invoke setup.py directly
• It's a good idea to clean-up a few directories before building packages
• Move the "twine check" step to after building the packages (it's not entirely clear to me what build-scripts/01-build.sh is for); also add --strict to catch warnings
• packaging-requirements.txt only needs to contain build and twine, because the other requirements are defined in the build-system of pyproject.toml

[mwtoews]

Looking at #88, it is trivial to specify "README.md" without needing anything fancy.

[larsbutler]

Hi @mwtoews, thank you very much for this contribution! Looks good to me.

Could you please add yourself to the AUTHORS.txt file?

And also, would you like to become a maintainer? (You can read about what that entails here: https://github.com/geomet/geomet/blob/master/CONTRIBUTING.md.) Just reply in the thread if you'd like that, and I'll set it up. :)

[larsbutler]

This is awesome. I've been meaning to do this work, and just haven't had the time. :D

Please add yourself to the AUTHORS.txt and I'm happy to merge this.