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>

subcmds/__init__.py

@@ -37,9 +37,7 @@ for py in os.listdir(my_dir):
         try:
             cmd = getattr(mod, clsn)
         except AttributeError:
-            raise SyntaxError(
-                "%s/%s does not define class %s" % (__name__, py, clsn)
-            )
+            raise SyntaxError(f"{__name__}/{py} does not define class {clsn}")
 
         name = name.replace("_", "-")
         cmd.NAME = name