tox.ini, constraints.txt: Lock the version of black to <24

The formatting produced by black versions before 24 matches the current
formatting of the code.

Change-Id: I045f22d2f32a09d4683867293e81512f2abd1036
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/434997
Tested-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

constraints.txt

@@ -0,0 +1 @@
+black<24

tox.ini

@@ -30,6 +30,7 @@ python =
 
 [testenv]
 deps =
+    -c constraints.txt
     black
     flake8
     isort
@@ -44,6 +45,7 @@ setenv =
 [testenv:lint]
 skip_install = true
 deps =
+    -c constraints.txt
     black
     flake8
 commands =
@@ -53,6 +55,7 @@ commands =
 [testenv:format]
 skip_install = true
 deps =
+    -c constraints.txt
     black
     flake8
 commands =