[Tests] Add reformat environment to tox

It calls black and isort on the source tree.

tox.ini

@@ -36,6 +36,11 @@ commands = poetry build
 [testenv:docs]
 commands = poetry run make -C docs/ html {posargs}
 
+[testenv:reformat]
+commands =
+    poetry run black biscuit/core/
+    poetry run isort -rc biscuit/core/
+
 [flake8]
 max_line_length = 100
 exclude = migrations,tests