Write version in pyproject toml if available

[mauritsvanrees]

• Added pyproject.toml and moved most metadata there. "Eat your own dog food." I am not sure if we can get rid of the remaining parts of setup.py.
• Add reader. I added tomli to the dependencies on Pythons earlier than 3.11. Chances are that you already have this via a different dependency. On 3.11 I use the built-in tomllib.
• Add writer. This is not that different from how we write to setup.py or other files.
• Checked that bin/prerelease updates the version in our own pyproject.toml. I did not commit this.

This fixes issue #373.

[mauritsvanrees]

Okay, so I added a test, and it works fine. The message from the commit may be interesting: 1bd131523698c383d6974ded8c55a95355b498c6.

Problem: the test does not work on Python 3.6, so I dropped support for that. We will want to make a major release for this. And the test shows that if you only have a pyproject.toml and no setup.py at all, then fullrelease does not create and upload distributions to PyPI. So that is something that we need to fix. That is basically issue #295.

I think the current PR is fine to merge: it enables updating the version when it is in pyproject.toml. But releasing still needs a setup.py at this point, though it can be mostly empty. We could make an 8.0.0a1 release with this.

[reinout]

I agree. I'll merge it.