fix: detection of PURE_PYTHON where environ sets '0'

E.g., our current 'tox.ini' does this, which effectively disables testing
the C extension.

src/zope/hookable/__init__.py

@@ -18,7 +18,7 @@
 
 
 _PYPY = platform.python_implementation() in ('PyPy', 'Jython')
-_PURE_PYTHON = os.environ.get('PURE_PYTHON', _PYPY)
+_PURE_PYTHON = os.environ.get('PURE_PYTHON') == '1'
 
 
 class _py_hookable:
@@ -69,7 +69,7 @@ def __call__(self, *args, **kw):
 except ImportError:  # pragma: no cover
     _c_hookable = None
 
-if _PURE_PYTHON or _c_hookable is None:
+if _PYPY or _PURE_PYTHON or _c_hookable is None:
     hookable = _py_hookable
 else:  # pragma: no cover
     hookable = _c_hookable