fix(setuptools): use LICENSE.txt

Author: villevsv-upcloud

setup.cfg

@@ -9,7 +9,7 @@ maintainer_email = hello@upcloud.com
 url = https://github.com/UpCloudLtd/upcloud-python-api
 packages=['upcloud_api', 'upcloud_api.cloud_manager']
 license = MIT
-license_files = []
+license_files = ['LICENSE.txt']
 
 [options]
 python_requires = >=3.9, <4