Adopt PEP 639 "license" field (#914)

* PEP 639 compliance

Keep the redundant Trove classifier for now, until tools such as
pip-licenses support PEP 639.

* No need to explictly specify LICENSE file

Indeed flit will automatically pick up files with standard names,
including `LICEN[CS]E*`:
https://flit.pypa.io/en/stable/pyproject_toml.html#new-style-metadata

* Bump flit to 3.11 for basic PEP 639 support

https://flit.pypa.io/en/stable/history.html#version-3-11

* flit 3.11 does not like the License classifier

flit_core.config.ConfigError: License classifiers are deprecated in favor of the license expression.
                              Remove the 'License :: OSI Approved :: MIT License' classifier

Author: DimitriPapadopoulos

pyproject.toml

@@ -1,21 +1,20 @@
 [build-system]
-requires = ["flit-core >= 3.8"]
+requires = ["flit-core >= 3.11"]
 build-backend = "flit_core.buildapi"
 
 [project]
 name = "build"
 description = "A simple, correct Python build frontend"
 readme = "README.md"
 requires-python = ">= 3.9"
-license.file = "LICENSE"
+license = "MIT"
 authors = [
   { name = "Filipe Laíns", email = "[email protected]" },
   { name = "Bernát Gábor", email = "[email protected]" },
   { name = "layday", email = "[email protected]" },
   { name = "Henry Schreiner", email = "[email protected]" },
 ]
 classifiers = [
-  "License :: OSI Approved :: MIT License",
   "Programming Language :: Python :: 3",
   "Programming Language :: Python :: 3 :: Only",
   "Programming Language :: Python :: 3.9",