Add missing files to sdist

Update the flit configuration to include documentation and test files
in source distribution again.  The flit defaults changed recently, and
the files are missing in 9.0.0 release.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

pyproject.toml

@@ -101,5 +101,18 @@ statistics = true
 ignore = "E202"
 exclude = "./data,./src,.svn,CVS,.bzr,.hg,.git,__pycache__"
 
+[tool.flit.sdist]
+include = [
+  "AUTHORS.md",
+  "CHANGELOG.md",
+  "CITATION.cff",
+  "conftest.py",
+  "docs/",
+  "mypy.ini",
+  "noxfile.py",
+  "tests/",
+  "uv.lock",
+]
+
 [tool.pytest.ini_options]
 addopts = "--pdbcls=IPython.terminal.debugger:Pdb"