Revert ":fire: Delete MANIFEST file"

This reverts commit 6b51403.

The commit never made sense. It deleted MANIFEST, but a couple years
later MANIFEST.in got added right back, which included the same files
MANIFEST did. And the commit added a data_files argument to setup.py
which was totally invalid -- it tried to install README.rst as
/usr/README.rst which was clearly wrong.

The original intention appears to have been to borrow the side effect of
data_files, and have the files appear in the sdist so setup.py could
read them, but it was the wrong way to do it.

Fixes: ikegami-yukino#28

setup.py

@@ -43,7 +43,6 @@
           'Programming Language :: Python :: 3.11',
           'Programming Language :: Python :: 3.12', 'Topic :: Text Processing'
       ],
-      data_files=[('', ['README.rst', 'CHANGES.rst'])],
       long_description='%s\n\n%s' %
       (open('README.rst', encoding='utf8').read(),
        open('CHANGES.rst', encoding='utf8').read()),