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

@@ -215,7 +215,7 @@ to update the working directory files.
 
             if not opt.quiet:
                 print()
-            print("Your identity is: %s <%s>" % (name, email))
+            print(f"Your identity is: {name} <{email}>")
             print("is this correct [y/N]? ", end="", flush=True)
             a = sys.stdin.readline().strip().lower()
             if a in ("yes", "y", "t", "true"):