Fix blank line issues reported by flake8

- E301 expected 1 blank line
- E302 expected 2 blank lines
- E303 too many blank lines
- E305 expected 2 blank lines after class or function definition
- E306 expected 1 blank line before a nested definition

Fixed automatically with autopep8:

  git ls-files | grep py$ | xargs autopep8 --in-place \
    --select E301,E302,E303,E305,E306

Manually fix issues in project.py caused by misuse of block comments.

Change-Id: Iee840fcaff48aae504ddac9c3e76d2acd484f6a9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254599
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>
diff --git a/subcmds/checkout.py b/subcmds/checkout.py
index c8a09a8..efa31d2 100644
--- a/subcmds/checkout.py
+++ b/subcmds/checkout.py
@@ -19,6 +19,7 @@
 from command import Command
 from progress import Progress
 
+
 class Checkout(Command):
   common = True
   helpSummary = "Checkout a branch for development"