The oldest still-supported version of Blender is 4.2-LTS, which uses
Python 3.11, so that's what's now the required Python version for BAT.
Make the build system depend on `poetry-core` instead of `poetry`, to
make package builds faster (especially important when using Tox for
testing in various environments).
This also re-generates the `poetry.lock` file, so updates the
dependencies to their latest version compatible with the `project.toml`
file.
BAT now can take advantage of the `zstandard` module to handle Blender
3.0+ compressed blend files.
If the module is not installed, the blend files cannot be opened but
GZip-compressed and uncompressed files can still be handled.
Python 3.5 is EOL already, and Blender never used version 3.6 (it went
straight to 3.7), hence 3.6 was never tested explicitly.
Python 3.7 or newer is required from now on.
