Include tox.ini in sdist (#120)

Fixes #117
diff --git a/pyproject.toml b/pyproject.toml
index 41bf2be..67d81a1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -57,5 +57,5 @@
 email = "levkivskyi@gmail.com"
 
 [tool.flit.sdist]
-include = ["CHANGELOG.md", "README.md", "*/*test*.py"]
+include = ["CHANGELOG.md", "README.md", "tox.ini", "*/*test*.py"]
 exclude = []