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.
This reverts commit 46466de7db924d374a97c74219db9694b9a67bbd.
Tox wasn't set up correctly for this bump, and we may want to keep
compatibility with older Blenders as well.
