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>

progress.py

@@ -52,11 +52,11 @@ def duration_str(total):
     uses microsecond resolution.  This makes for noisy output.
     """
     hours, mins, secs = convert_to_hms(total)
-    ret = "%.3fs" % (secs,)
+    ret = f"{secs:.3f}s"
     if mins:
-        ret = "%im%s" % (mins, ret)
+        ret = f"{mins}m{ret}"
     if hours:
-        ret = "%ih%s" % (hours, ret)
+        ret = f"{hours}h{ret}"
     return ret