PR: Map exec_ to their non-deprecated alternatives

[kumattau]

This PR fixes #286.

All exec_s of PySide6 are simply mapped (assigned) to execs to avoid the following DeprecationWarning of PySide6.

DeprecationWarning: 'exec_' will be removed in the future. Use 'exec' instead.
  sys.exit(app.exec_())

I think if there is no problem if exec_ is overwritten, this PR can be merged.
If a way to access the original exec_ is required, QtPy needs to provide the option for controlling the overwrite or the special attribute for accessing original exec_.

[dalthviz]

Thanks @kumattau for all the help with this one! LGTM 👍

[CAM-Gerlach]

Thanks @kumattau !