tags: use modern // operator for division

Fixes a test on Python 3.

# skip-blame only correcting a division operator, not a substantive change

Differential Revision: https://phab.mercurial-scm.org/D8108

mercurial/tags.py

@@ -721,7 +721,7 b' class hgtagsfnodescache(object):'
         self._dirtyoffset = None
 
         rawlentokeep = min(
-            wantedlen, (rawlen / _fnodesrecsize) * _fnodesrecsize
+            wantedlen, (rawlen // _fnodesrecsize) * _fnodesrecsize
         )
         if rawlen > rawlentokeep:
             # There's no easy way to truncate array instances. This seems