fix: Use new license file name (#55)

The license file name changed from
LICENSE to LICENSE.txt, and it wasn't updated
in the metadata.

Author: Christopher-Chianelli

setup.py

@@ -123,7 +123,7 @@ def find_stub_files(stub_root: str):
     name='timefold-solver',
     version=timefold_solver_python_version,
     license='Apache License Version 2.0',
-    license_file='LICENSE',
+    license_file='LICENSE.txt',
     description='An AI constraint solver that optimizes planning and scheduling problems',
     long_description=long_description,
     long_description_content_type='text/markdown',