py: PEP 639 compliance

* Update licensing metadata to match PEP 639.
* I don't know where the ISC license classifier comes from, but the
  rest, both the LICENSE file and the SPDX-License-Identifier at the
  top of pyproject.toml only refer to the MIT license.

Author: DimitriPapadopoulos

pyproject.toml

@@ -14,16 +14,15 @@ name = 'pkgconf'
 version = '2.5.1-0'
 description = '`pkgconf` is a program which helps with discovering library dependencies and configuring compiler and linker flags.'
 readme = 'README.md'
-license = {file = 'LICENSE'}
+license = 'MIT'
+license-files = ['LICENSE']
 authors = [
   {name = 'Ralf Gommers', email = '[email protected]'},
   {name = 'Filipe Laíns', email = '[email protected]'}
 ]
 classifiers = [
   'Development Status :: 4 - Beta',
   'Intended Audience :: Developers',
-  'License :: OSI Approved :: MIT License',
-  'License :: OSI Approved :: ISC License (ISCL)',
   'Programming Language :: C',
   'Topic :: Software Development',
   'Operating System :: Microsoft :: Windows',