Adopt PEP 621, move metadata into `pyproject.toml`

[Rongronggg9]

https://github.com/MagicStack/asyncpg/pull/900#issuecomment-1079653458

Support for declaring project metadata or configuring setuptools via pyproject.toml files is still experimental and might change (or be removed) in future releases.

As @abravalheri advised, we should either drop the [project] table (#903) or fully adopt PEP 621 (this PR). However, since the support for pyproject.toml of setuptools is still experimental and requires a fairly high setuptools version, we'd better wait at least until it stabilizes and asyncpg officially drops the compatibility with Python 3.6. I will keep tracking on this.

If the maintainers are never willing to adopt PEP 621 even in the foreseeable future, just close this PR.