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>

color.py

@@ -194,7 +194,7 @@ class Coloring:
         if not opt:
             return _Color(fg, bg, attr)
 
-        v = self._config.GetString("%s.%s" % (self._section, opt))
+        v = self._config.GetString(f"{self._section}.{opt}")
         if v is None:
             return _Color(fg, bg, attr)