fix: v3bwfile: Add method to set under_min_report KeyValue

in the Bandwidth Lines.
When the minimum percent of relays it's not reached, still report
the relays but set KeyValues accordingly so that Tor doesn't vote
on their bandwidth.

Part of #29853.

sbws/lib/v3bwfile.py

@@ -977,6 +977,17 @@ class V3BWFile(object):
                           key=lambda l: l.bw)
         return cls(header, bw_lines)
 
+    @staticmethod
+    def set_under_min_report(bw_lines):
+        """
+        Mondify the Bandwidth Lines adding the KeyValue `under_min_report`,
+        `vote`.
+        """
+        log.debug("Setting `under_min_report` to %s lines.", len(bw_lines))
+        for l in bw_lines:
+            l.under_min_report = 1
+            l.vote = 0
+
     @staticmethod
     def bw_kb(bw_lines, reverse=False):
         bw_lines_scaled = copy.deepcopy(bw_lines)