cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

tests/test_git_command.py

@@ -19,12 +19,7 @@ import os
 import re
 import subprocess
 import unittest
-
-
-try:
-    from unittest import mock
-except ImportError:
-    import mock
+from unittest import mock
 
 import git_command
 import wrapper