Switch Python 3.12 to non-future and change with-future-python behavior

[dataflake]

This PR does two things:

• move Python 3.12 to the list of standard non-future Pythons
• prepare places that want to use future Pythons by using py313
• redefine FUTURE_PYTHON_VERSION to an empty string
• only render future python-related template items if FUTURE_PYTHON_VERSION is not empty

With the last two changes individual packages do not need to turn off their with-future-python flag during those times when the Python release is finished and no prerelease exists for the next Python version.

[dataflake]

A manual test on the DocumentTemplate repo shows that the only change individual packages will need for "official" Python 3.12 support is adding the trove classifier in setup.py.

[icemac]

I like the changes.
The first alpha of 3.14 is due to be released in about 2 weeks, so we are prepared.