pyproject metadata updates, and minor docs fix

[kounelisagis]

Fixes readinto() description for the docs and moves more metadata into pyproject.toml.

[dudoslav]

I am getting lost in what file is used for building this repo. Is it setup.py or pyproject.toml? Or both and it is then combined? What would happen if setup.py would define the same variables as pyproject.toml? like version_scheme for example.

[kounelisagis]

I'm trying to move everything to .toml. Some things depend on Python functions, so it's more complex.

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"While it is not technically necessary yet, it is STRONGLY RECOMMENDED for a project to have a pyproject.toml file at the root of its source tree"
"One way to look at it is that the scope of setuptools has now been reduced to the role of a build backend."
https://packaging.python.org/en/latest/guides/modernize-setup-py-project/#modernize-setup-py-project

[dudoslav]

I would recommend to also get @ihnorton 's approval.

[ihnorton]

Small doc changes are fine to merge with ✅ from anyone. Just make sure to put build system change in the title too in order to get attention on the scope of the change - ie I would suggest titling this PR something like "pyproject metadata updates, and minor docs fix".