Make black with line length 80 repo's code style

Provide a consistent formatting style and tox commands to lint and
format.

Bug: b/267675342
Change-Id: I33ddfe07af8473f4334c347d156246bfb66d4cfe
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/362954
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

tox.ini

@@ -15,7 +15,7 @@
 # https://tox.readthedocs.io/
 
 [tox]
-envlist = py36, py37, py38, py39, py310
+envlist = lint, py36, py37, py38, py39, py310
 
 [gh-actions]
 python =
@@ -35,5 +35,23 @@ setenv =
     GIT_COMMITTER_NAME = Repo test committer
     EMAIL = repo@gerrit.nodomain
 
+[testenv:lint]
+skip_install = true
+deps =
+    black
+    flake8
+commands =
+    black --check {posargs:.}
+    flake8
+
+[testenv:format]
+skip_install = true
+deps =
+    black
+    flake8
+commands =
+    black {posargs:.}
+    flake8
+
 [pytest]
 timeout = 300