Log Message:
Update gobject-introspection to 2.60.1

Highlights:
* build: Drop Python 2 support, require Python 3.4+ :mr:`69`
* Add _Float128 to the base C types
* scanner: rework source root directory guessing code to not depend on the
  build directory
* scanner: Merge specifiers and qualifiers when merging basic types.
  Fixes "unsigned char" being wrongly parsed as "unsigned" etc.
  :mr:`125` (:user:`Tomasz Miąsko <tmiasko>`)
* scanner: make using bool without stdbool include work again
  :issue:`247`