fix: Python 3 compatibility and C++ code quality improvements

[shbhmexe]

Proposed Changes

This PR fixes Python 2 to Python 3 compatibility issues and improves C++ code quality.

Files Modified:

Module	File	Changes
SU2_PY	SU2/util/bunch.py	.iteritems() → .items(), keys().sort() → sorted(keys())
SU2_PY	SU2/util/ordered_bunch.py	.iteritems() → .items()
SU2_PY	SU2/util/ordered_dict.py	.itervalues() → .values()
SU2_PY	SU2/io/data.py	unicode → str
SU2_PY	SU2/util/switch.py	raise StopIteration → return (PEP 479)
SU2_CFD	src/solvers/CSpeciesFlameletSolver.cpp	Remove redundant == true

Issues Fixed

Python 3 Compatibility:

Issue	Impact	Fix
.iteritems() removed in Py3	AttributeError	Use .items()
.itervalues() removed in Py3	AttributeError	Use .values()
dict.keys().sort() fails	AttributeError	Use sorted()
unicode type removed	NameError	Use str
raise StopIteration in generators	RuntimeError (PEP 479)	Use return

C++ Code Quality:

Issue	Impact	Fix
== true comparison	Redundant code	Direct boolean check

Related Work

This PR extends the Python 3 compatibility work started in PR #2640, addressing remaining issues in utility modules.

PR Checklist

• [x] I am submitting my contribution to the develop branch.
• [x] My contribution generates no new compiler warnings.
• [x] My contribution is commented and consistent with SU2 style.
• [x] I used the pre-commit hook to prevent dirty commits.
• [ ] I have added a test case that demonstrates my contribution, if necessary.
• [ ] I have updated appropriate documentation, if necessary.