Include tox.ini in sdist to fix tests

Include the `tox.ini` file in sdist, in order to fix a few test failures
due to an additional error message printed by tox, i.e.:

    FAILED tests/config/cli/test_parse.py::test_verbosity_guess_miss_match - AssertionError: assert 'ROOT: No tox.ini or setup.cfg or pyproject.toml found, assuming empty tox.ini at /tmp/portage/dev-python/t...
    FAILED tests/config/loader/test_loader.py::test_override_incorrect[-x] - AssertionError: assert not 'ROOT: No tox.ini or setup.cfg or pyproject.toml found, assuming empty tox.ini at /tmp/portage/dev-pyth...
    FAILED tests/config/loader/test_loader.py::test_override_incorrect[--override] - AssertionError: assert not 'ROOT: No tox.ini or setup.cfg or pyproject.toml found, assuming empty tox.ini at /tmp/portage/dev-pyth...

docs/changelog/2939.bugfix.rst

@@ -0,0 +1 @@
+``tox.ini`` is now included in source distributions in order to make all tests pass.

pyproject.toml

@@ -100,7 +100,7 @@ scripts.tox = "tox.run:run"
 [tool.hatch]
 build.dev-mode-dirs = ["src"]
 build.hooks.vcs.version-file = "src/tox/version.py"
-build.targets.sdist.include = ["/src", "/tests"]
+build.targets.sdist.include = ["/src", "/tests", "/tox.ini"]
 version.source = "vcs"
 
 [tool.black]