Include license file in sdist

[hugovk]

Fixes https://github.com/python/python-docs-theme/issues/122.

With flit-core, looks like we need to explicitly include the licence file:

license

A table with either a file key (a relative path to a license file) or a text key (the license text).

https://flit.pypa.io/en/stable/pyproject_toml.html

With setuptools, the 2022.1 sdist includes:

-rw-r--r--   1 hugo  staff   2.4K Mar 22 15:34 LICENSE
-rw-r--r--   1 hugo  staff    67B Mar 27 17:44 MANIFEST.in
-rw-r--r--   1 hugo  staff   1.4K Mar 27 17:44 PKG-INFO
-rw-r--r--   1 hugo  staff   858B Mar 22 15:34 README.rst
-rw-r--r--   1 hugo  staff   105B Mar 27 17:44 pyproject.toml
drwxr-xr-x   7 hugo  staff   224B Mar 27 17:44 python_docs_theme
drwxr-xr-x   7 hugo  staff   224B Mar 27 17:44 python_docs_theme.egg-info
-rw-r--r--   1 hugo  staff    67B Mar 27 17:44 setup.cfg
-rw-r--r--   1 hugo  staff   1.2K Mar 27 17:44 setup.py

With flit, the v2023.3.1 sdist includes:

-rw-r--r--  1 hugo  staff   2.0K Jan  1  1970 PKG-INFO
-rw-r--r--  1 hugo  staff   858B Mar 22 15:34 README.rst
-rw-r--r--  1 hugo  staff   1.4K Mar 27 17:44 pyproject.toml
drwxr-xr-x  7 hugo  staff   224B Mar 27 17:47 python_docs_theme

With this PR, the sdist includes:

-rw-r--r--  1 hugo  staff   2.4K Mar 22 15:34 LICENSE
-rw-r--r--  1 hugo  staff   2.0K Jan  1  1970 PKG-INFO
-rw-r--r--  1 hugo  staff   858B Mar 22 15:34 README.rst
-rw-r--r--  1 hugo  staff   1.4K Mar 27 17:47 pyproject.toml
drwxr-xr-x  7 hugo  staff   224B Mar 27 17:48 python_docs_theme