Update project metadata

[cdce8p]

Convert "old" metadata from [tool.flit.metadata] to [project]. https://flit.pypa.io/en/stable/pyproject_toml.html

Also add explicit license information and the license file to the sdist + wheel.

Project metadata diff (with flit 3.11):

 ...
-Author: Pradyun Gedam
-Author-email: [email protected]
+Author-email: Pradyun Gedam <[email protected]>
+License-Expression: MIT
 License-File: LICENSE
-Requires-Dist: sphinx >= 3
+Requires-Dist: sphinx>=3
-Home-page: https://github.com/pradyunsg/sphinx-inline-tabs
+Project-URL: Documentation, https://sphinx-inline-tabs.readthedocs.io
+Project-URL: Homepage, https://github.com/pradyunsg/sphinx-inline-tabs
 ...