Qcore

- Fix version number

(Not released to PyPI because of a mistake.)

- Drop support for Python 3.8; add support for Python 3.13

* Add minimum version requirement (Python 3.8) to metadata
* Build Python 3.12 wheels

* Drop support for Python 3.7
* Add support for Python 3.12
* Support building with Cython 3

* Pin to `Cython<3` to fix build

* Drop support for Python 3.6; add support for Python 3.11
* Add functions `utime_as_datetime`, `datetime_as_utime`,
`format_utime_as_iso_8601`, `iso_8601_as_utime`