Drop Black target version

It can now be automatically inferred from pyproject.toml: https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html\#t-target-version

Author: adamchainz

pyproject.toml

@@ -48,9 +48,6 @@ Repository = "https://github.com/adamchainz/time-machine"
 [project.entry-points.pytest11]
 time_machine = "time_machine"
 
-[tool.black]
-target-version = ['py38']
-
 [tool.isort]
 add_imports = [
     "from __future__ import annotations"