…rovements

Python fixes:
- Replace deprecated .iteritems() with .items() in bunch.py,
  ordered_bunch.py
- Replace deprecated .itervalues() with .values() in ordered_dict.py
- Replace keys().sort() with sorted(keys()) in bunch.py as
  dict.keys() returns a view object in Python 3
- Replace unicode type with str in data.py (unicode was merged
  into str in Python 3)
- Replace 'raise StopIteration' with 'return' in switch.py
  (PEP 479: raising StopIteration inside a generator is deprecated
  in Python 3.7+ and causes RuntimeError)

C++ fixes:
- Remove redundant '== true' boolean comparison in
  CSpeciesFlameletSolver.cpp (line 443)

These changes ensure the SU2 Python utilities work correctly with
Python 3.x and improve C++ code quality.

Signed-off-by: shbhmexe <shubhushukla586@gmail.com>