Modify tox to show diff of formatting changes with black

With this change PR submitters can see what needs
to be changed instead of just knowing that
black needs to be run on the code. I think this
is a good kind of feedback

1 files changed, 1 insertions, 1 deletions

diff --git a/tox.ini b/tox.ini
index 9ea794983..ed9ea4f9d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -39,7 +39,7 @@ deps =
     black==20.8b1
     isort==5.5.2
 commands =
-    black --check . --exclude="tests/functional/|tests/input|tests/extensions/data|tests/regrtest_data/|tests/data/|venv|astroid|.tox"
+    black --diff --check . --exclude="tests/functional/|tests/input|tests/extensions/data|tests/regrtest_data/|tests/data/|venv|astroid|.tox"
     isort . --check-only
 changedir = {toxinidir}