Add tox.ini to sdist (#15853)

Fixes #14142

Author: mtelka

MANIFEST.in

@@ -39,9 +39,10 @@ graft test-data
 include conftest.py
 include runtests.py
 include pytest.ini
+include tox.ini
 
 include LICENSE mypyc/README.md
-exclude .gitmodules CONTRIBUTING.md CREDITS ROADMAP.md tox.ini action.yml .editorconfig
+exclude .gitmodules CONTRIBUTING.md CREDITS ROADMAP.md action.yml .editorconfig
 exclude .git-blame-ignore-revs .pre-commit-config.yaml
 
 global-exclude *.py[cod]