Update string formatting to f-strings in examples and fix getObjVal()

[adamj34]

Replaces format() with f-string formatting, which I think is a bit nicer to read, especially in examples.

Before updating docs/tutorials/eventhandler.rst I ran it just out of curiosity, and surprisingly I got this lovely error:

Traceback (most recent call last):
  File "src/pyscipopt/scip.pxi", line 2908, in pyscipopt.scip.Model.attachEventHandlerCallback.EventHandler.eventexec
    self.callback(model, event)
  File "/home/nu/PySCIPOpt/play.py", line 4, in print_obj_value
    print("New best solution found with objective value: {}".format(model.getObjVal()))
                                                                    ~~~~~~~~~~~~~~~^^
  File "src/pyscipopt/scip.pxi", line 11633, in pyscipopt.scip.Model.getObjVal
    assert self._bestSol.sol != NULL
AssertionError
Exception ignored in: 'pyscipopt.scip.PyEventExec'
Traceback (most recent call last):
  File "src/pyscipopt/scip.pxi", line 2908, in pyscipopt.scip.Model.attachEventHandlerCallback.EventHandler.eventexec
    self.callback(model, event)
  File "/home/nu/PySCIPOpt/play.py", line 4, in print_obj_value
    print("New best solution found with objective value: {}".format(model.getObjVal()))
  File "src/pyscipopt/scip.pxi", line 11633, in pyscipopt.scip.Model.getObjVal
    assert self._bestSol.sol != NULL
AssertionError: 
[event.c:385] ERROR: Error <0> in function call
[event.c:2443] ERROR: Error <0> in function call
[event.c:1847] ERROR: Error <0> in function call
[primal.c:1083] ERROR: Error <0> in function call
[primal.c:1881] ERROR: Error <0> in function call
[scip_sol.c:4075] ERROR: Error <0> in function call
[heur_trivial.c:169] ERROR: Error <0> in function call
[heur.c:1319] ERROR: Error <0> in function call
[solve.c:377] ERROR: Error <0> in function call
[scip_solve.c:1165] ERROR: Error <0> in function call
[scip_solve.c:2458] ERROR: Error <0> in function call
[scip_solve.c:2710] ERROR: Error <0> in function call
Traceback (most recent call last):
  File "/home/nu/PySCIPOpt/play.py", line 8, in <module>
    m.optimize()
    ~~~~~~~~~~^^
  File "src/pyscipopt/scip.pxi", line 9360, in pyscipopt.scip.Model.optimize
    PY_SCIP_CALL(SCIPsolve(self._scip))
  File "src/pyscipopt/scip.pxi", line 325, in pyscipopt.scip.PY_SCIP_CALL
    raise Exception('SCIP: unspecified error!')
Exception: SCIP: unspecified error!

I guess this is related to #993? Either way, I fixed it in a similar way to how it was done for getVal() there.

[Joao-Dionisio]

Ahhh I have such a love-hate relationship with both these methods and PySCIPOpt's error handling.

But everything seems correct with your changes! Could you please add a test where this fails without the fix (and similarly for the bug from #993)?

And thank you thank you!

[adamj34]

I've added four new tests that would be failing if the fix was absent from both these methods.

[Joao-Dionisio]

Thank you so much @adamj34 !