pygtksourceview: New formula (2.10.0)

- New formula pygtksourceview with test (fixes #16410)
- Fix the pkgconfig file of pygtk, too, because the
  GTK project moved the `codegen` executable from
  pygtk into pygobject.
- Fix minor typo in pygtk.

Fixes #16410.
Closes #16418.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>