Remove undeclared dependency on past.builtins

[Abhayindia]

Fixes #1252.

dronekit/__init__.py imports basestring from past.builtins, but python-future (which provides past) was never listed as a dependency in setup.py. A fresh pip install dronekit fails immediately on import.

basestring is only needed for Python 2/3 compatibility, so this replaces the import with the same sys.version_info gating already used a few lines up for the collections.abc change - no new dependency needed.

Confirmed the module is genuinely gone and the import still works:

$ python -c "import past"
ModuleNotFoundError: No module named 'past'

$ python -c "import dronekit; print('import OK')"
import OK