Add LICENSE, AUTHORS and ChangeLog to PyPI package (closes #15054)

MANIFEST.in

@@ -1,4 +1,7 @@
 include README.md
+include LICENSE
+include AUTHORS
+include ChangeLog
 include test/*.py
 include test/*.json
 include youtube-dl.bash-completion

setup.py

@@ -109,6 +109,7 @@ setup(
     author_email='ytdl@yt-dl.org',
     maintainer='Sergey M.',
     maintainer_email='dstftw@gmail.com',
+    license='Unlicense',
     packages=[
         'youtube_dl',
         'youtube_dl.extractor', 'youtube_dl.downloader',