Drop a Python 2 compatibility hack and fix handling of bool properties in GaudiPython

Going from type(v) is int to isinstance(v, int) in !1415 (merged) changed the way GaudiPython handles bool properties, because, surprisingly,

>>> isinstance(True, int)
True
>>> type(True) is int
False

Since the check was actually meant only for the special Python 2 type long and we do not support anymore Python 2, dropping that check fixes the issue reported in #276 (closed).

Closes #276 (closed)