Avoid implicit int conversion with vertical tabs

qutebrowser/mainwindow/tabwidget.py:645: DeprecationWarning: an
  integer is required (got type float).  Implicit conversion to integers
  using __int__ is deprecated, and may be removed in a future version of
  Python.
    size = QSize(width, height)

This was a left-over from 66ab7f2634.

See #4928

qutebrowser/mainwindow/tabwidget.py

@@ -639,7 +639,7 @@ class TabBar(QTabBar):
                 main_window = objreg.get('main-window', scope='window',
                                          window=self._win_id)
                 perc = int(confwidth.rstrip('%'))
-                width = main_window.width() * perc / 100
+                width = main_window.width() * perc // 100
             else:
                 width = int(confwidth)
             size = QSize(width, height)