build: Add LICENSE.txt file

Author: boriel

pyproject.toml

@@ -6,6 +6,9 @@ authors = [
     { name = "Jose Rodriguez", email = "zxbasic@boriel.com" }
 ]
 license = "AGPL-3.0-or-later"
+license-files = [
+    "LICENSE.txt"
+]
 readme = "README.md"
 requires-python = ">=3.11"
 keywords = ["compiler", "zxspectrum", "BASIC", "z80"]