tox.ini: add isort as dependency

a previous introduced isort, which causes tox
runs to fail for all python versions. adding
isort as dependency resolve these issues.

Change-Id: If3faf78e6928e6e5111b2ef2359351459832431f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/384175
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

tox.ini

@@ -30,6 +30,7 @@ python =
 deps =
     black
     flake8
+    isort
     pytest
     pytest-timeout
 commands = {envpython} run_tests {posargs}