Use "exec()" instead of "exec_()" with PySide6

[rahulporuri]

Like enthought/pyface#1179, we should aim to use exec() with PySide6 instead of exec_() because the latter is deprecated.

C:\Users\rporuri\.edm\envs\<env-name>\lib\site-packages\traitsui\qt4\ui_base.py:320: DeprecationWarning: 'exec_' will be removed in the future. Use 'exec' instead.
  ui.control.exec_()

We see the above deprecation warning when we run our testsuite. The deprecation warning is coming from

traitsui/traitsui/qt4/ui_base.py

Line 320 in ce8b285

ui.control.exec_()

There are a number of similar uses of exec_() in the codebase. For the record, enthought/pyface#1208 is a more or less accepted solution at the moment to handle the use of exec() vs exec_().