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

@@ -43,9 +43,6 @@ Changelog = "https://github.com/adamchainz/django-linear-migrations/blob/main/CH
 Funding = "https://adamj.eu/books/"
 Repository = "https://github.com/adamchainz/django-linear-migrations"
 
-[tool.black]
-target-version = ['py38']
-
 [tool.isort]
 add_imports = [
     "from __future__ import annotations"