Drop Python 2 compatibility branches across OMV runtime/install paths

[Copilot]

OMV still contained Python 2-era conditionals in core install and execution paths despite Python 2 no longer being supported. This PR removes those branches and normalizes behavior to Python 3-only code paths.

• Engine install path cleanup

  • Removed NetPyNE’s Python 2-only dependency preinstall branch in omv/engines/getnetpyne.py.
  • Simplified NEURON default version handling in omv/engines/getnrn.py to Python 3 assumptions, with a single default constant (DEFAULT_NEURON_VERSION).
  • Added explicit version-family matcher for pip-based NEURON installs (7.8.x, 8.x) via _supports_pip_install(...).

• Runtime execution path cleanup

  • Removed Python-version branching around subprocess input encoding in:
    • omv/engines/neuron_.py
    • omv/engines/pyneuron.py
  • Both paths now always send encoded command bytes (Python 3 behavior).

• Parser/output simplification

  • Removed redundant Python 2/3 conditional formatting in omv/parse_omt.py where both branches were identical.

• Legacy generated CI config alignment

  • Updated autogen Travis template target from Python 2.7 to 3.12 in omv/autogen.py.

• Focused regression coverage

  • Added omv/test/test_drop_py2_bits.py to lock in:
    • default NEURON install version behavior,
    • supported pip-install version-family behavior,
    • NetPyNE install path behavior without Python 2 branches.

Example of the simplification:

# before
if sys.version_info[0] == 3:
    c = dedent(cmd).encode()
else:
    c = dedent(cmd)

# after
c = dedent(cmd).encode()