Stop using the deprecated "license_file" option

Setuptools has replaced this with the "license_files" option.

pyproject.toml

-# NOTE: setuptools 40.9.0 added support for projects without setup.py.
+# Setuptools 40.9.0 added support for projects without setup.py.
+# Setuptools 42.0.0 added support for the "license_files" option.
 
 [build-system]
 requires = [
-    "setuptools >= 40.9",
+    "setuptools >= 42",
     "wheel",
 ]
 build-backend = "setuptools.build_meta"

setup.cfg

@@ -9,7 +9,7 @@ description = Epidemic forecasting with mechanistic infection models
 long_description = file: README.rst, NEWS.rst
 long_description_content_type = text/x-rst
 license = BSD 3-Clause License
-license_file = LICENSE
+license_files = LICENSE
 author = Rob Moss
 author_email = rgmoss@unimelb.edu.au
 platforms = any