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>

.flake8

@@ -1,15 +1,10 @@
 [flake8]
-max-line-length=100
-ignore=
-    # E111: Indentation is not a multiple of four
-    E111,
-    # E114: Indentation is not a multiple of four (comment)
-    E114,
+max-line-length = 80
+extend-ignore =
+    # E203: Whitespace before ':'
+    # See https://github.com/PyCQA/pycodestyle/issues/373
+    E203,
     # E402: Module level import not at top of file
     E402,
     # E731: do not assign a lambda expression, use a def
     E731,
-    # W503: Line break before binary operator
-    W503,
-    # W504: Line break after binary operator
-    W504