Restore Python bindings on current Python

[sobolevnrm]

Closes #300.

Summary

• update the vendored pybind11 dependency to v3.1.0
• use vendored pybind11 when no installed CMake package is available
• replace obsolete embedded PyString_* calls in the SWIG interface
• install the SWIG module normally instead of copying into a sibling PDB2PQR checkout
• initialize and share the pybind input parser implementation
• update the legacy drivers for Python 3
• add a Python 3.14 build/import/parser smoke test for both binding implementations

Validation

• both _apbslib and apbs_pybind configure, build, and install locally with Python 3.14.7 and SWIG 4.5.1
• both installed modules import and parse the real legacy INPUT string successfully
• legacy main.py and noinput.py import successfully
• Python suite: 205 passed, 5 skipped, 2 xfailed, 1 xpassed
• independent read-only review found no blockers